Shouldn't dogs that attack humans be put down in any case? I thought that would be standard consequence if a dog bites a person.
Unfortunately many police forces have no training in dogs, and so officers don't know the difference between an aggressive attacking dog and a playful bouncing dog, and thus shoot them.
EDIT: It's especially unfortunate if the dog has been tasered and is safely caught in a noose at the end of a stick (http://www.denverpost.com/opinion/ci_22895415/police-dog-han...), or if the police have visited a family to tell them that their son has died, and then they shoot his dog while they're there. (http://www.timesdispatch.com/news/henrico-police-shoot-pet-a...)
Hard to tell that until it's too late.