Only thing we know is this guy has a bruise on his back, no reason to assume the cop aimed and shot at his back. People sometimes get kicked in the face by police horses and videos show that they got behind the horse, fucked around and found out. If you believe random assholes on the internet with only an injury as proof, I've got a totally safe vaccine to sell you.
They have the right to do it; it's for a good cause.