True. But that doesn't keep them from trying.
A "gun free zone" is a place where only law-abiding citizens are carefully disarmed. It doesn't take much thought to see a major flaw in this reasoning.
This particular case happened quite rapidly--in a matter of a few seconds--and it's likely that even had some people in the area been armed, it may have made little difference. Where it's tragic is those cases where a gunman wanders leisurely about for some time, killing his cowering and helpless victims at whim.