"Doesn't make a difference now, as there wasn't any additional "bombers"...but... they didn't know diddely squat last night"
They knew an explosive device of some kind had detonated outside the stadium, so I can see why a logical response was to keep the crowd from going outside the stadium. Beyond that, they were dealing with an unknown.
Do you think an evacuation of that stadium would have been safe and orderly, had there been an announcement that one bomb had gone off outside on one end of the stadium, and they did not know if other explosive devices were present?
In hindsight, they made the right decision. Had it been me locked down in there, I might feel differently, as you do. But, their response worked.
The threat with these types of situations involves secondary devices, which are a very real possibility if this really was a terrorist event. The kid obviously got his big bang before he was ready, or maybe he didn't.
I wonder if there were secondary devices planted around some of the other exits....not all of them...just a few would do the job. The kid detonates his pack, he plants a few secondary devices at a few key exit points in anticpation of an evac being ordered for the stadium. Perhaps the secondary devices would be detonated with a timer or some kind of motion sensor. In any event, he doesn't have to kill everyone, justa few to create a real panic exodus where many would be hurt and/or killed. I think the kid detonated before he was ready. Hopefully the reason was good security around the stadium...but maybe he was just having a change of heart...whatever the case we can be thankful the kid was the only one offed when that bomb pack exploded.
I have spent alot of time in training from the ATF/FBI as a first responder for bombing incidents....the worst part is that damned secondary device sweep....you don't want to find a device the hard way.