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?
It's useful for an imperialist ruling class to keep drug trafficking and drug traffickers around, it's possible that some are powerful enough that they are effectively on the same level. We're not so far removed from things like British opium imports to China having a huge effect on quality of life and such. They can disrupt other nations and be used as a pretext for sanctions/occupations/foreign aide to facilitate god knows what.
For domestic operations it's slush money and can be politicized. Its illegality puts people in situations where they'll be more likely to do anti-social actions benefiting or on behalf of clandestine operations. It's also a great cover story for any violence that might erupt between elements of this, for lack of a better term, "deep political network".