Capitalism has a deep-seated taboo against taking recreational drugs. So strong is the taboo they will ruin your life and exile you from mainstream society for doing something recreational.
This is changing a bit as the scientists tell them there is basically no reason for this. But the scientists meet with resistance from entrenched cops, judges, lawyers, who are very 
about it.
What's the materialist explanation for this moralistic taboo?
On top of what's already been mentioned here, the CIA have operated what is likely the largest drug trafficking operation in the world since as early as the 1950s (pretty much since the agency's inception). Busting on competition also serves to artificially increase market prices so they can rake in more dark money for their off the books operations. (See Politics of Heroin in South East Asia - D. Valentine)