because of the possibilityof ambush.
many terrorist events now are designed to get the troops there and once in attendance they are fired up from outside the arena. The bus in this case could havebeen only an initiator.
I guess in this case the gooseheads, knowing it is training exercise had a good cause to believe there was not going to be any extraneous fire so they had a gandewr at the main action. One would hope in the real thing they stay focused on the task at hand.