Some libertarians are so hung up on US foreign aid and meddling that they overlook the real reasons behind the attack. I think its pretty stupid and short sighted of them and not all libertarians see it that way.
I am libertarian, and I understand perfectly that 9-11 was not due to anything the US has done or not done, but because of who Americans are and what we believe in. Some people don't want to believe that anyone would hate us personally so they look for excuses.
I think so too, they hate us for who we are. In fact, hatred seems deeply imbedded in much of their culture.