Wel I think I agree on the prison thing. I have heard people say 'why should druggies get their treatments free, I have to pay for my healthcare. Well dude you're paying more for them to be in prison. I got into a huge argument with my BIL about it actually. just after I'd moved here.
Part of the problem is that treating someone for addiction as you've said involves far more than simply making them stop taking something. If that were the case, it'd be easy. But if you live in an area with high and easy drug availability, and are known to be a user, then you are targetted by them again as soon as you get back. Studies have shown the most successful way to get off and stay off drugs is to get away. But that's hugely expensive as an intervention. People aren't prepared to accept that.
The treatment that gives the best results from a social point of view; reduction in crime, reduction in harm, ability to hold down a job, increase in literacy and interaction with education is long-term methadone treatment. Yeah, drug users think methadone is like cooking chocolate in comparison to heroin which is more 70% cocoa dark goodness, but long-term studies show it's damned effective.
Politically, it's the poor relation. Which leaves health care providers giving poorer care than they know they can to people who aren't really interested in the treatment being offered. More jail, more $$$, more deaths, and the politicians don't care.