It was my impression that, under the US system, those convicted of crimes related to drug use are very often required to go to therapy.
(Guards who are overworked and underpaid)
Who brings them in? Possibly guards but visitors can also bring in a lot. Pretrial detention is tougher. The drugs are brought into prison and then smuggled into pretrial detention.
Edit: I'd never seen anything except weed before I went to jail. Inside jail I saw and was offered every single drug known to man in whatever quantity I wanted.
"TBS" is "prison with psychiatric care". It's a bit complex because you first get your punishment (prison term) and then after that additional TBS treatment (until you're "safe to be released", basically).
I'm not claiming this is representative for Europe, or even the rest of the Dutch prisons. However, everything else I've seen over the years from most European countries gave me the impression the average is closer to that than what I've seen/heard from US prisons.
Solitary confinement on its own is already controversial enough, but being pushed in a cell naked and literally nothing to do?! What the actual fuck?!