They are taught when at the academy to shot the dog when confronted. It does two things. 1. eliminates a potential threat and 2. Shows everyone else just who is in charge at the scene.
I will bet that it was part of the officers initial training and part of the ongoing training at the local department.
It does two things:
1. Shows everyone they are out of control and cowardly to an effeminate level.
2. Demonstrates undebatably to the public that they lack the judgement to detirmine what a threat is.