It's what keeps him tough in everyone's mind. See, every other sheriff in America houses their inmates in an actual building, but not The Toughest Sheriff in America. He chooses to keep them in tents which can get up to 125 degrees in the summer.
Meanwhile, he brags about keeping his pound animals in an air conditioned building, and he spends more to feed dogs than he does the human beings he's supposedly responsible for.
I'm sorry, but to me it's inhumane, immoral, and totally unnecessary to treat people that way. Especially since approximately 70% of the "inmates" in Tent City haven't even been to trial yet. Isn't the punishment supposed to come after guilt has been established?
Most of the inmates are in custody for misdeameanors, which is why they haven't gone to trial yet - they're not exactly murderers or rapists, so there's no hurry. In fact, they're downright useful to the Joke - they help him keep his reputation as "TTSIA," since Tent City is his claim to fame.