Good thing it wasn't a gladiator type breed.
Reality is the cop didn't want to shoot the Aussie in front of the 4 year old. If it was a pit , it likely wouldn't have been a little nip/retreat based on their nature. As mentioned Aussies can nip but retreat. This is what they may do to a sheep.. Nip to send a message to move them along, then retreat to avoid a kick in the skull. If this were a proper attack, the bat would have come out. Or the pepper spray. Or the chunk.