What I know about prisons in the US comes from watching prison documentaries, which just makes me shake my head in disbelief over how ridiculously ineffective those prisons are. It seems to me that they are effectively race segregated, and the lives of the prisoners is mainly ruled by race based gangs prisoners have to join for their own safety -- as gang wars in prison are common -- even if they don't particularly want to.
US prisons are segregated on the basis of sex, class, race, threat level... At some point someone has to notice how fine-grained the segregation has to be to keep some semblance of security and come up with "what if we just keep all dangerous people from all the other dangerous people, so that they don't constantly have to fear all the dangerous people..."