You've missed the part where drugs become addictive.
Random citizen's thought process:
1. Huh, heroin is legalised
2. Hmm, just got paid, might see what the fuss is all about
3. "Hello shopkeeper, some heroin please"
4. I might take that heroin now
5. Fuck, that feels good, definitely getting some more of that
6. Fuck that feels good, I need more of that
7. Might skip work today, do some heroin.
8. Might quit work, do some heroin. Can't really concentrate, I'm so high
9. Damn, no income this week for my heroin. Oh well, better go and do crime.