But, since they did not know where it might have been heading...and even if it was a small plane - it could have crashed into some office window, caused a fire or enough damage to harm/hurt, even kill someone. Just protecting one life makes it worthwhile to evacuate, IMHO. I would not want to be in the one place that even a small plane purposely crashed.
I had a small plane fly so low over my house last year that I actually got up and ran downstairs. I was not a happy camper. I thought it was going to come through an upstairs window.