I think it's understandable, given the restrictions that go on in prison. Any movie that depicts sexuality is a hard sell. Even if there's different opinions on how graphic it is, there is sex in the movie and they tend to not want to show it to inmates. I'm not an expert and I don't work in the prison system, but I know people who have been in jail and it's pretty strict to dumb comedies or very mainstream, non-offensive stuff.
I don't agree with the thinking because it treats all inmates as though they couldn't handle depictions of sex, but that's my understanding of why they wouldn't show BBM.
Juan