I would assume the group homes call the cops because of child rights. Every foster child and I assume every group home child knows "his rights." And given the many ears willing to listen to discontent children most staff at these establishments can't be bothered with the hassle. If you're going to allow group home staff to take care of problems they should know they'll be backed up and not threatened with a law suit.
It really has gotten out of hand in Canada. The justice system and the various arms that nestle it have broken down under the weight of endless bureaucracy. Just a joke.