Ok, read the article. As one who's studied this sort of mayhem for some time and currently makes a living mitigating said mayhem for the US military, my advice would be not to ignore authority should they be giving you reasonable instruction, instruction that made sense to you. Obviously, in hindsight, staying put in the towers was a mistake but had the circumstances been a bit different it may have been the right thing to do. Suppose the firefighters could have gotten upstairs to put the fire out but were blocked by fleeing inhabitants? We all know now that it wouldnt have helped but had the fire been less intense it may have been possible to put it out. The authorities that make these calls today spend many hours training and studying the very scenarios it is their charge to protect you from. They're not always right, but given the same level of information, they're judgement is likely to be better than yours.
If the cessna that penetrated the adiz had been carrying a large old artillery round smuggled in through canada or stolen in the US and detonated it as it crashed into the capital building, well, they have something like a 50 or 100 meter casualty radius so you do the math. Theyre the same devices the bad guys are using for ied's all over iraq. They're everywhere. Any idea how many landmines are out there? How hard do you think it would be to dig a few up, use the mil grade explosive to make a really nice 500lb bomb and fly it into a building. Ya ever been on the canadian border? There's nothing for miles and miles and plenty of cover in some places. Very easily crossed. Anyway, in that scenario its best to get people out of the target area. The use of chemical or biological weapons being spread by aircraft, in my view, is a distant 2nd place given the difficulty in weaponizing the stuff and distributing it effectively. So, given that the most likely scenario of an explosives type attack the authorities did the right thing. If they were really smart they would have ushered the people into routes that took them upwind just in case. Anyway, my 2 cents.