The American regular forces can fight in the continental US but only if the country is invaded by an enemy army, or if the President invokes the War Measures Act (I'm not sure what it's called in the US) which would be very unpopular with the American people and would likely spawn more retaliatory acts against the government.

Terrorists are clearly unafraid of high-end technology since it has not proven to be particularly effective in either locating or defeating them.

It's the fact that there are only a small number of terrorists and that they operate, for a good part, independently of each other that makes them so difficult to find and eliminate. It's a bit like looking for a needle in a haystack. Besides, once they find them, how do they gain access to them - Osama bin Laden in Pakistan is a example of how terrorists manage to evade prosecution by hiding in countries that are anti-American.

Besides who is the terrorist? The assumption now is that they are Islamic fundamentalists but I would point out that the second largest act of terrorism commited in the US was commited by an American citizen.