It's messed up, but my guess is simply that they're trying to limit prisoner communication with the outside world and they don't want the general public to see how they're run.
The thing is that when you invoke 'they' you don't mean Facebook. Facebook as a corporation doesn't care about the general public seeing how prisons are run.
The government - and where it shares coffers with private prison moguls - cares.
The government and the private prison industry are able to project their goals through Facebook.
