Compare past and present terrorists. In the past, terrorists had some cause or wanted something. They used violence as a means to some end. (Much as we do with our military, but let's not muddy the issue. If we attack and kill with our military, that's OK because we're right and they're wrong.)
In the usual scenario, something real bad happened and about the time everybody was wondering who done it, the message would come from some mysterious spokesman...
"Yeah, that was us. We committed this heinous act because you did ____ to us this was our way of getting even. If you don't stop doing ____ and do ____, then more innocents will get the same."
Our present terrorists are so modest that they won't even come forward to take credit and make demands. (All we get is a long string of doctored and faked messages.) Bin Laden has denied all along any connection with 911 and the only evidence we have been offered are transparent fakes. The standard of evidence needed for war should be high, right?
And what if it really was Bin Laden? Shouldn't we just go in and get him without taking out the whole Middle East? We might even have a trial and find out whom he was connected to and what his problem was. Well... on the other hand, maybe that's not such a good idea.
It makes no sense for terrorists to attack and then deny it without coming forward with a list of demands. If there's nothing to be gained from the terror and if it brings the wrath of the world down upon you, then what's the point?
Maybe one of the reasons the public believes the official 911 story is because deep down they know why terrorists might want to attack us. It's un-American and embarrassing to talk about this sort of thing so let's just pretend that they hate our freedom and our magnificence, our decadence and our three HumVee garages.