There are very few topics I can't say this with. South Park summed it up best. Those writers have a way of sending up complex political and social issues in absolutely ridiculous parody plots that somehow make a lot of sense.
Basically, all the evidence that supports the 9/11 Conspiracy was put out by the American Government... the President and his group even went as far as having evil villain speeches, rubbing their hands together and talking about 'beautiful money'.
Really, this entire conspiracy was propagated by the Government so they wouldn't look stupid. I mean, what's worse, one of the most powerful nations having its heart torn out, or 'evil master planning President blows up his own Trade Centre and blames it on Muslims so he can steal their oil'. It made them look powerful and in control.
Kyle asked "Wait, so who was responsible for 9/11 then?" Stan replied. "Duh, a bunch of pissed off Muslims" The Hardy Boys added "Yeah, what are you, retarded?"
I'd recommend watching the episode. Its got some bloody great lines and common sense.