The Conductor Conundrum
I want to go on record as saying that I take a rather dim view of the theory, which has sprung up among some adult fans inclined to "read between the lines," that Stan Shunpike was indeed guilty of collusion with the Death Eaters. This, I think, is jumping to a totally unsupported conclusion which, if true, would undermine other parts of Rowling's apparent theme regarding the despicable corruption of her world. If these fans have managed to convince themselves of Stan's guilt on the basis of what little we're shown in the books, I sincerely hope that none of them have anything to do with the legal system, including jury duty.
Now, I don't deny that the Death Eaters would have a lot to offer someone of a very obviously underprivileged background, stuck in a low-paying dead-end job. I also don't deny that, as the most well-known (possibly the only) conductor of a popular transportation service, Stan would have had useful information to offer the Death Eaters himself. And finally, neither do I deny that Harry has a very self-righteous streak that often clouds his ability to see past his own biases (as in the case of Snape). But that's not evidence of guilt, any of it. The first two would be motive, not evidence, and I am inclined to suspect that this sector of the fans got so put out with Harry's perennially incurious nature (and his author's equally aggravating tendency to play "because I said so" instead of giving good reasons) that they decided to go against anything he said that wasn't demonstrably true. But all the evidence we are given suggests that this is exactly what Harry thinks it is, and that Stan originally was put under the Imperius Curse because he was so useful. The one bit of dubious information is the detail in the sixth book that he was arrested in a pub while "talking about Death Eater plans." However, this could easily mean a botched Imperius Curse. (It could also mean something far nastier regarding the Ministry, which I will get to in a second.) For heaven's sake, the last time we see him, in midair in the aerial chase, his face was blank-looking. The man was under the Imperius Curse. He was not guilty. Get over it.
Now, what's that nastier possibility? Well, I am going to get political briefly. Forgive me. But it is fairly commonly known that the wealthy and well-connected are much more likely to evade the law than the poor nobodies. A great many bureaucratic "investigations" into incompetence and wrongdoing have concluded with the blame pinned upon some poor soul at the bottom rung of the organizational ladder, clearly to protect the higher-ups who proverbially fiddled while Rome burned and wouldn't let the lower echelons do what needed to be done, or worse, who actively directed bad policy. Some fine examples of this are the Hurricane Katrina and BP oil spill aftermaths (and a dishonorable mention for the Abu Ghraib torture investigation). Scapegoating is common, and the type of person most likely to be a victim of such is a poor nobody who doesn't have the resources to defend himself or herself. Somebody exactly like Shunpike, who, as the Trio noted, would be the very type of person to mouth off in a way that would grab the attention of anyone who was looking for a patsy to pin something on or to bring in as a "success story." And we have been given plenty of examples of how a world of magic is much more prone to frame-ups and scapegoating than ours. We do not need to hypothesize that Stan was truly, knowingly, aiding the Death Eaters, and the evidence of both Stan's own appearances as well as the pervasive corruption of Wizarding Britain strongly supports Harry's opinion of the matter.
Once we accept the Imperius Curse, this invites a new question, however. If Stan was Imperiused, why was he picked to go on that adventure? (And the whole adventure was pointless, based on what we were shown. There is absolutely no reason why someone couldn't have Apparated Harry out of that house. Apparition cannot be tracked. If there were wards put up by the Ministry preventing it, why weren't we told that?) Why was he broken out of Azkaban in the first place? What purpose would the Death Eaters have for picking an extremely weak wizard who was under the Imperius Curse—and apparently a subpar curse, since it showed in his face?
And Stan was a weak wizard. It is all but impossible not to conclude that he was a Hogwarts dropout, though not at all for the same reason as Harry. His given age indicates that he should have been there at least for Harry's first two years if he had gone the full seven years, and he obviously hadn't. He didn't even recognize Harry when he got on the bus. He had dropped out after his fifth year, his O.W.L. year. He probably did have some O.W.L.s, but not high enough scores to progress to the higher-level N.E.W.T. classes. And yet, he would have passed those O.W.L.s, so there would be nothing for him to gain by taking the remedial classes that are mentioned in the sixth book for some sixth- and seventh-year students to take in order to get their O.W.L.s. year or two late. We also have the example of Marcus Flint, who attended school for eight years. It was originally an authorial error, but this is the official explanation now, so we're stuck with it. Evidently he was a true dim bulb who couldn't get his O.W.L.s until the fourth try, because I seriously doubt that he would be allowed to repeat on account of failing to get his N.E.W.T.s. Anyway, I imagine these "remedial classes" are just the fifth-year O.W.L. classes, actually, rather than a separate class for the higher divisions. But either way, Hogwarts has nothing more to offer those students who pass their O.W.L.s but don't pass high enough to be admitted to the N.E.W.T. class. It's like getting through college with a 2.00 GPA. You won't be admitted to graduate school, but you've got your degree. You are "qualified," as it were, to look for a great many more jobs than before.
But, again: Why would the Death Eaters have put him in that operation, especially with a subpar Imperius Curse in the picture? Unfortunately, the likely real-world answer is Rowling saying "because I needed Harry to give himself away somehow," but let's see if something can be put together that fits.
Well, we know that Yaxley, a Death Eater, put Pius Thicknesse, the Head of the Department of Magical Law Enforcement at the time of the chase, under the Imperius Curse. We know that Yaxley was high enough in the Department of Magical Law Enforcement (and/or close enough to Thicknesse) that he was tapped to replace him without any questions asked when Thicknesse became Minister. These people were in the Ministry during Scrimgeour's brief tenure as Minister, and since there is no evidence to the contrary, we can assume that Thicknesse had been the Departmental Head during Harry's entire sixth year. Now, what did Harry accuse Scrimgeour of in the sixth book with respect to Stan Shunpike's imprisonment? Being like Bartemius Crouch Sr., wasn't it? And Crouch was... the head of the Department of Magical Law Enforcement. What bureau in the Ministry would handle imprisonments? Right.
Scrimgeour had a bad temper and little discretion regarding the separation of the personal and the professional spheres (witness his piggybacking on Percy Weasley's Christmas visit to get a word with Harry). At the time, he would not have supposed that his Department Heads were untrustworthy. He could easily have complained to them about Harry. And it's possible there may have been a longer association between Scrimgeour and Thicknesse. Let's back up a bit. Scrimgeour, the Head Auror, was picked to be Minister in all likelihood because Amelia Bones, the DMLE Head during Harry's fifth year, was killed by Voldemort. I rather suspect she was originally supposed to replace Fudge. The Minister and DMLE positions opened up at the same time because of Fudge's resignation and Bones's murder; Thicknesse became DMLE Head when Scrimgeour became Minister. I doubt he answered to Scrimgeour before Scrimgeour became Minister, but his position before being named Department Head was probably comparable in authority.
As a hunch, I'd say he headed the Improper Use of Magic Office and was in tight with Dolores Umbridge, who, as you will notice, got a nice promotion (and was not punished or even investigated for her conduct during the fifth year. Someone high up in the Ministry was looking out for her). That division is one of Rowling's perennial bureaucratic villains. They went after Harry for Dobby's cake levitation charm and his own Patronus. They were all set to go after "Moody" for the exploding trash incident, and Amos Diggory says anxiously that Moody has "had it" if they get involved. And if my theory about the Trace is correct, that office was specifically targeting only Muggle-born or Muggle-raised children with the threat of expulsion, preventing only them from keeping their skills honed, putting only them at risk of being denied the opportunity to become qualified wizards, and that this was the original despicable purpose of the Trace rather than any rubbish about protecting Muggles. The Improper Use of Magic Office is effectively the "authoritarian surveillance state" apparatus of the Ministry. The Imperius Curse would have plenty to work with on someone with that mindset. I am starting to think that the curse comes off better if its victim already legitimately has the seeds of whatever behavior the controller wants to produce. Besides, we should also look at the man's name. Rowling's names are not subtle in the least, and we have known that since the first book. "Pious and thick." Sounds a lot like the attitude of the Improper Use of Magic officials.
At that level of bureaucracy, Thicknesse and Scrimgeour certainly could have regarded each other as suitable people to associate with. That's not at all uncommon in real-world government bureaus. But whatever kind of association Scrimgeour had with Thicknesse, Thicknesse and Yaxley certainly worked closely enough together that Yaxley was able to put him under the Imperius Curse and replace him when he became Minister. Yaxley was probably a career civil servant and another of Voldemort's long-standing Ministry moles. He was almost certainly sabotaging Thicknesse's office as soon as Thicknesse got named to be a Department Head, if not before then. With the Scrimgeour-Thicknesse-Yaxley (or just Scrimgeour-Thicknesse) connection, as well as the known temperament of Scrimgeour, there is plenty of support for a game of "telephone" regarding Harry's complaints about Shunpike, and it is no stretch whatsoever to say that the Death Eaters got wind of them.
So the Death Eaters already knew, probably from Yaxley, possibly from Thicknesse by that time, that Harry had taken on Shunpike's wrongful imprisonment as a personal cause and a rhetorical weapon to use against the Ministry. They put Shunpike in the chase as a trap, and Harry fell right into it. It's a lovely little parallel with what Voldemort did during Harry's fifth year, using his concern for Sirius Black to lead him into a trap. Harry really ought to wise up to this.
Ah, but wait, you might say. The Death Eaters didn't know that there would be six Polyjuiced Harry Potters in the sky. And you're right; they didn't. But it seems reasonable that they would have expected some form of disguise or concealment. They also knew that they were not supposed to kill Harry; Voldemort was supposed to do that, but they had free rein to kill all the Order members that they could manage. If there was a disguise, then obviously they would need some way to sniff Harry out. So put someone in the chase that Harry would not want to attack with violence, and direct that Imperiused puppet to go after him. They know his signature spell. It's dead easy.
And it is beyond me why any fan would want to theorize that Shunpike was indeed involved with the Death Eaters. The Imperius Curse, upon its very first introduction, was said to have caused a lot of trouble for the Ministry. Between the fall of the Crouch family, the control of Madam Rosmerta for Death Eater ends, the puppeting of Pius Thicknesse, and the use of Shunpike as bait for Harry, we can see for ourselves just what a problem Imperius really is. We don't need to take anyone's word for it. In this one little plot thread is a perfect example of just how much trouble the Imperius Curse can cause even when the victim is not socially or politically "important." You don't have to be the Minister for Magic for the Imperius Curse to cause problems if it is cast on you. Everyone's free will is important. I don't know why any fan would want to throw that point out in favor of assuming Shunpike's guilt when so little is gained in exchange.