The American government did not set it up. The sheer thought of the government doing something that well is a trip into insanity land. It's incapable of doing anything. Also, I bet you a million dollars if it happened if Gore was in power, you wouldn't think the government did it.
What the American government did was not prevent it. They knew it was coming, just as they had the warnings for the underwear bomber and the Fort Hood shootings recently. There were warnings, but no one took them up on it. It's how s*** like this happens.
What purpose would setting up 9/11 serve? We aren't in a police state; we have the patriot act, but it's not half as bad as it is in modern Russia. The two wars have made certain companies a lot of money, as well as many politicians, but it's ruined what little trust people have in the government: so they set it up for short term monetary gain? I don't buy it.
Buying into "the government did it" is pointless anyways. You should be smarter then that.
I will say the CIA and the FBI are terribly run, bloated, broken, and unaccountable.