Hmmm, I am not trying to troll or start a flame war here, but yes it is. Granted, I am not an American but in the corrective system here in Australia, where I work, yes it is. Prisoners are not allowed to have belts as they pose as a weapon and their pants are made from loose elastic.
On the flip side, most of them get too fat after being in jail that they can't fit into the clothes that they came in with. Granted, there are some prisoners who are just blatant in their attempts for some 'love', but generally speaking, the crims who walk around that way are either plastic gangsters (ie, wannabe's) who don't really know what they are doing, or looking for a heavy (a crim with a position of authority, perceived or otherwise) to take them under their wing and are offering the only thing they have (their ass).