Unlike every other cities' jail in the game, Riften Jail is not owned by a crime faction that belongs specifically to city guards. Instead, it is owned by CrimeFactionRift, which includes every NPC in The Rift Hold (including Fort Dawnguard NPCs). Maybe this is deliberate for some reason, but if you become friends with one or more NPCs in The Rift you can freely take most items without it counting as theft.
There is no GuardFactionRiften, while there is a GuardFaction<insert city name> for each of the other eight hold capitals. As far as I know this isn't causing any significant problems in the game, but it does seem bizarre that you almost certainly will be able to steal low value items from the jail without consequences at some point in the game.