This is just an extension of standard police procedure in most places, most of the time. They don't want to lose cops, so they keep them out of danger. But in this case, the nutjob was actively shooting little kids. It's not a situation where you can wait and evaluate -- you have to rush in immediately. The cops fucked up. A head is going to roll because of this, probably that of police chief Peter Arredondo. He will be a sacrifice, to appease the fury of parents.