What you also have to think is of the situation the police are in right now. Disregard the fact a kid was killed, disregard the race issues for now.
They are under massive pressure. The whole world is watching. For some reason, the US Government has thought it would be a great idea to give police departments surplus military gear such as MTP helmets, rifles, APCs etc. Police don't need this ****. Police don't WANT this ****. Who is going to need a camo helmet/pants in the middle of a city? They are probably all pulling double if not triple shifts to provide the amount of manpower the Governor wants on the streets right now. Anything and everything they do is being scrutinized by the international media, and by their own command structure.
They are also fearing for their lives. Yes, an unarmed kid was killed. But riots are a whole nother kettle of fish. They are unpredictable, and absolutely anything can happen. You can get singled out and dragged away, you can get set on ****ing fire, and some people in St. Louis have actually said they want to go out and kill police officers. You are up against an unknown number of people who can come at you from an infinite amount of directions, with unknown weapons, dressed in anything. Someone has a camera? So what? They could also have a gun. You aren't going to shoot them, but you want them as far away as possible so they don't pose a threat and they don't get in your way. The only people you can trust in those kinds of situations are your buddies next to you, and I would tell everybody to back the **** off who wasn't a police officer if I was in that situation.
If that was you, would you not be on edge?