If so in Ontario, that is still a matter for a civil tort suit from those injured by the plow truck driven by the thief, and of course there is the whole issue of comparative negligence/non-absolute contributory negligence liablity as a defence by the plow company and their employees, since certainly the majority of the liablity remains with the thief.The thief's actions are separate from the claim that the drivers were negligent.
It is an offence to leave your car idling unattended. It is even an offence to leave your keys in the car, locked or not.