So, do you drive a manual, or an automatic? I'm under the impression that manuals don't have the "Don't go anywhere even if started" park detent that automatics do, and thus the parking brake is mandatory. It's as commonly called the "emergency brake" as it uses a secondary cabled system to actuate the brakes and not the power assist, so can serve in an emergency if the normal assist fails. For automatic transmissions, the parking / emergency brake is useful for not getting stuck in the park detent if on a hill, or reinforcing it on said hill just in case.
If said prat hit hard enough to break the parking pawl so it can roll, it's most certainly not the owner's fault for not having the parking brake engaged also, it's the prat that hit them and damaged the car, pushing it onward and the like.