A couple of points to consider, though:
1. Again: in polls like this many people are often really just indicating that they recognize the possibility in question as a logical possibility. To say that x is logically possible is just to say that x is possible on the broadest possible construal of 'possible.' That is, that x is not absolutely impossible, i.e. that x is non-self-contradictory.
Hopes for this defense may be fading, though. Apparently those polled indicated that they believed that involvement of the American government in 9/11 was 'very likely' or 'somewhat likely.' And that's quite different from 'logically possible.' Though people often don't really understand what they're saying when they say such things.
2. Though it doesn't really make things that much better, the article I link to above seems to suggest that it's anger at the government that is driving conspiracy theories, not the other way around.
Though 9/11 conspiracy theories are spookily irrational, they may be an irrational reaction to actual nefarious activities by the Bush administration. That is, roughly: people may finally be coming to recognize how angry they ought to be at this administration...but that anger may be coming out in the wrong form. Instead of being angry at the fact that the Bushies exhibited a willingness to steal an election, they go whole-hog and latch onto wacky 9/11 conspiracies. By the standards of the American public, exhibiting a willingness to steal an election is a terribly abstruse and ephemeral thing. Since nobody knows for sure whether they actually stole it (and since there may not even be a fact of the matter), it's not the kind of thing most people get angry about.
Similarly with Iraq. The administration was careful to muddy the waters enough to give themselves plausible deniability. Their lies were generally in the form of exaggerations and misplaced emphasis. Again, something that might be too fuzzy for most people to riled up about.
So (if the above is right), and if people really are fairly simple-minded about such things...and if they're really, really agry...and if they let their emotions interfere with their evaluations of likelihoods (which we know they do), then...mix all this up and...maybe...you get an explanation for this apparent insanity.
Very speculative...and doubly suspicious because so much of it coheres so well with my views about the other (i.e. non-9/11) issues...but there's a kind of first go at producing a tangle of hyptheses.
Sort of the punchline: the Bush dead-enders may try to spin this as: 9/11 irrationality causes anti-Bushism.
But my first guess, for what it's worth, is: Bush's awfulness causes 9/11 irrationality.
[HT: Statisticasaurus rex]
[O.k., stand by...
I skipped right over the following part:
The level of suspicion of U.S. official involvement in a 9/11 conspiracy was only slightly behind the 40 percent who suspect "officials in the federal government were directly responsible for the assassination of President Kennedy" and the 38 percent who believe "the federal government is withholding proof of the existence of intelligent life from other planets."
So this may change things considerably. The real point to be made here may be:
We already have plenty of evidence that Americans are a little loony...so these 9/11 numbers shouldn't really come as a big surprise. Remember: more Americans believe in ESP than believe in evolution. So maybe this information just poignantly illustrates a general fact we should have been worried about all along: that Americans are kinda crazy.]