Putting aside all of the tinfoil-hattery for a second and let's assume the US government was behind the attacks. Why the hell would they go to the trouble of hijacking planes and flying them into buildings when they could just plant bombs instead?
a) When has the US needed an excuse to declare war?
b) Would blowing up the World Trade Centre with everyone in it not be enough to cause public outcry then?
Ignoring all the conspiracy theories for a second: if you were the US government and were planning a "false flag" attack on your own people, why the hell would you go the trouble of flying passenger planes into a building? Why not just plant some explosives, blow it up, arrest a couple of Middle...