Because your sorry butt marches into our emergency room after you screw yourself up. Taking away extra nice things that could have been. And since we can't drop you off on a deserted island to get rid of you for damaging the rest of us, the next best thing is to try to influence your behavior so you don't damage yourself now.
Its the same thing you do when you find a skunk roaming the house, you either get a broom and shoo it away, or restrict its freedom so at least it doesn't spray anything. What the fuck gives you the right to put the skunk in a box? The right of the others to remain un-sprayed by twitchy tailed skunk.
Should I not bike in my free time, because I might hit a pothole, endo, and break my arm? That would march "[my] sorry butt" into "[your] emergency room" pretty darn quick.
The answer is: that's absurd. Every single thing humans do can result in injury. (Even sitting in a chair injures you!) Cycling probably sends more people to the emergency room than smoking pot does, in fact.
(I don't use marijuana BTW, but I do support its legalization. Sounds like a legitimate recreational activity. I personally prefer coffee.)
Its the same thing you do when you find a skunk roaming the house, you either get a broom and shoo it away, or restrict its freedom so at least it doesn't spray anything. What the fuck gives you the right to put the skunk in a box? The right of the others to remain un-sprayed by twitchy tailed skunk.