A dog attack is not much different than being attacked by a human with a knife. You can stand there and try to judge their intentions but have to react fast if charged otherwise you are in serious trouble. If an armed thug charges a cop, I hope the cop is able to draw and shoot him before he is attacked. No different than with a dog.
People and dogs ARE different.
People have the ability and inclination to disguise their intentions.
Dogs make their intentions clear to anyone taking the time (a few seconds) to read them.
A dog is not comparable to a man armed with a knife. You are trying to justify this officer.