Two questions:
1. What was the motive for the government to blow up the WTC, the Pentagon, and farmground in PA?
2. If the govt. had shot down all 4 planes using F-18's would there be similar "truther" type stuff? This of course we need to be answered in the context of whatever the answer to question 1 is.

1. Well lets see here, we started two wars that we had no business getting into and now were about to start one with Iran. Also, the government can pass any legislation they want now to control us and people think it's ok because it's meant for the "terrorists". See the patriot act.
2. That's a really dumb question
Oh okay, the motive was to start some wars and to create legislation to "control us". Wait, why did they want to do that?
What does not shooting down the planes change? (seemingly important part of the conspiracy is this NORAD bit)
Why did they need planes in the first place, couldn't they just impload the building (like some muslim tried in the 90's) the same way you said they did, then blame it on said muslims?
Was the government going for style points with the 747's while at the same time trying to look ignorant by not shooting them down?