The dog was not in an aggressive mood, tail wagging and all. Appears to me that the cop simply panicked. If the dogs were a concern for him, he should have asked the owner to take them inside or something.
Tough call. But, a wagging tail is not a guarantee of a dog being happy to see you. And in the video, it does look as if the hair on the dog's back is raised and quite visible as such. Just sayin'...tough call. But it is easier to hold fire until actually threatened. Then no one can refute it.