If person X wants to kill themselves, you're right - that's there problem. But when the USA find that they need to pay for hospital care, or X killed Y, that's a different matter entirely. That's endangering, or "using" another person to pay for themselves. Then there's what happens when they get hooked, and they have to steal, kill, or some other offence. Can you say rehab?
I now think that SOME drugs can be legalized (again - with a HARD test or something) but not all. Decriminalization would be good on some drugs, but on plants that make hard drugs, what about those?