holy: I believe gumboot and others have addressed this fairly well, but I'll add what I know to the situation as well. As I stated, I work within the intelligence community (hereafter abbreviated IC) and have brought up the question of reactions and foreknowledge to many intel professionals. I've even spoken directly with people who were high up in the FAA heirarchy that day and are now working for TSA or other agencies. All of them, when asked about what they thought THEN (as opposed to now) stated unanimously that the idea of planes being flown into buildings was, to them, a ludicrous theory. Up until that point, nearly every hijacked aircraft had landed safely, and the hijackers nearly always intended to use the passengers as hostages for some monetary or political gain (i.e. a large ransom payoff or the release of a political prisoner). There were a few small isolated examples of hijackers smuggling explosives on board and blowing up the plane, and I believe one foiled attempt to hijack an aircraft heading to Paris in order to ram the Eiffel Tower with it (I'll have to see if I can locate the source on that though; I'm probably remembering it wrong. If anyone can help, please do so! My google-fu is not so good), but no strong indications (PRE-9/11 that is; POST-9/11 it was pretty obvious that some things people had dismissed as unlikely were clear indications of the plot to take place) that hijackers intended to turn planes into missiles and attack buildings. Taking that into account, can you understand why the actions you are positing did NOT take place then, but would if such an event were to occur again? 9/11 exposed a HUGE, GINORMOUS fault in our security, and we've been attempting to fix it since. As gumboot stated, should something similar happen now, the ideas you've posited would happen, or at least similar ones would, and it is much more likely that any such attempt would fail (although there's always a chance it would succeed, as no system is perfect). Do you understand what we mean now?
And by the way, if I came across as condescending or rude, please know that was not my intent; I simply want to address your concerns and hopefully mitigate them, but I recognize that the written word does not always convey the emotions intended properly.