I think the OP had it right, regardless of the details: it's your weed, you're responsible for it. Like anything else.
As for "why the ER?", having gone through what could be called an amicable divorce, I can tell you that even in that situation, if something happened to my kid and my ex even remotely disagreed with it, the ER would be high on her list of places to visit, just to put it on paper. Point being, there are too many variables to question that aspect of it.
I do say, though, that while using this scenario as the argument for more legislation is, as was already said, weak, the argument that "XYZ is just as dangerous" is equally weak. That killing you by torture and mutilation is a gruesome way to go doesn't mean I can kill you in a more humane way. While I'm for legalization of marijuana (I'd actually go further, but that's me) I also think that it is likely that research will show that much like alcohol, the "benign" effects of marijuana are likely only on a developed brain, and that young children are harmed by pot use in the same way that they are harmed by alcohol use. There is a condition known as "fetal alcohol syndrome" for a reason, you know, and estimates are that to a 16-year-old (and up, to as late as early 20's) brain, substance abuse is harmful in a way that it is not for fully mature (physically) adults.