My understanding was that it was frowned upon (in some families) because they’re more likely to flip when they have a long sentence hanging over them. Also, I thought I’d heard in some of the movies that they didn’t want their people dealing with drugs for fear of them getting addicted and losing their ability to do what they need to do (WRT the mafia).
If movies and TV shows have taught me anything, it’s that even when the boss says no drugs, there’s so much money in it for them, they do it anyway, as a side hustle.