And how is rushing people from several building out into the streets making them any safer? As suggested above, the plane could have just been the decoy for a ground operation. Even those kids who pulled the fire alarm in that school shooting a couple years ago were smart enough to think of that one.

People seem to be so concerned about what's going on in the sky that they don't think about the ground. e.g. The reports that the long security lines at LAX put people in more danger from terrorism, not less.

Lest we forget, creating a hole in the fuselage of an aircraft (a confined space, no less), is different than attacking a building.