You're kid can die while you're playing WoW too. Parent's who are trying to care for kids on heroin are negligent, but they were probably negligent before and after the heroin. When did you become the arbiter of which drugs were ok, and which were just too potent for the proletariat?
If, through negligence, you cause harm, well, that was already addressed by our legal system. Until there is a direct victim, a person who was harmed by an action, that action isn't a problem. And mandatory treatment? Maybe we should mandate treatment for people who drink too? After all that's addictive and can have consequences. Why don't we start treating people like adults: assume that they're capable of making their own decisions, and hold them accountable when (and only when) they actually do harm to others?