<Why ensure the worst-case scenario, that is, the airplane crashing into a downtown area midst the overspray of whatever they shot at it.

Flashed on the famous chase scene from the new, improved "Godzilla" movie (the one where Godzilla was buff instead of a big, fat, rubber lizard). Military pilots shot the living hell out of Manhattan while chasing the now-nimble giant reptile through the city streets. Never laid a glove on him, but wiped out dozens of buildings with their missiles. Was a lot funnier before 9-11.

Even if an aircraft is identified as terrorist, shooting it down is no easy decision. One would have to be over an uninhabited area, certainly not over a city (can you guarantee that the crashing plane would create fewer casualties and damage than if it hit its target?). And if the plane was full of innocent passengers, the decision gets even stickier, although if it's established that the pilot is a suicidal terrorist you've pretty much written them off anyway.