[andrewb]

I see a lot of comparing with weed/alcohol. As I see it you can lock people up for misusing alcohol to a point where they are becoming a danger to others.

As for weed, you can't lock people up for damaging themselves mentally (except in extreme cases), hence it's illegal to try and stop people getting in that situation.

I guess this all depends on your personal philosophy. Is the state there to protect your liberty from being invaded by others, or does it go further, should they be able to interfere in your own personal private liberty (kind of 'We know best')? Your answer to that question should sway you either way in the legality argument, but you'll find that your other views on things may conflict with your decision, if you apply the liberty argument elsewhere.