Okay, I'll throw you a bone, if you'll excuse the pun.
There was a clear and immediate danger to public safety due to these particular dogs and the owners lack of control over said dogs.
Therefore the police had a duty to stop the dogs and possibly thier owner, from wreaking further havoc.
The end result would be the dogs in question being put down anyway.
Why would the police let it go?
They shouldn't and they didn't.
It wasn't pretty and no one likes to see death, but these particular dogs would more than likely have gone on to bite or maim more people or other dogs if allowed to continue.