I think it's a good move by the cop. When you, as a small group of people, have to control large masses of people, it is important to constantly remind them that the slightest hesitation to obey commands will be met with extreme physical violence.
In this situation you have one of the small group assert physical domination by a completely unprovoked physical assault. Some times the big dog has to occasionally physically attack the smaller dogs to assert physical dominance. Even though the big dog relies on all the many small dogs for pack-protection and to provide sustenance, the small dogs must be kept in constant fear and servitude in order for the big dog to maintain unquestioned pack leadership.