Why didn't they just freaking kill it?
Probably couldn't with witnesses (including an animal control officer) standing right there.
Anyway, finishing off the wounded aggressor is a bad habit to get into. Never know who is going to be filming you with a video camera or one of those video cell phones thingys. Better to just kill them outright in the initial engagement. If you don't, you've got to treat them, dog or human.
Of course, I'm writing this as a retired Marine. Police have a different set of rules they must follow. Everytime I watch "COPS," I am amazed by their restraint.
I believe that was their intent, don't ya think?