>the dog would be killed by the police if it could be proved that it had bitten a human

and here lies the main issue - the dog gets punished for what is mainly the owner's fault (it is somewhat similar to destroying the gun in case of killing while letting the killer go free). I'm generally against corporal punishment, and this is one of the few exceptions where i think corporal punishment would be very useful - any damage inflicted by your dog as a result of your gross negligence should be inflicted as a punishment on you.