Unless they are specially-made tablets that do not have a WiFi chip, they have the capability to be connected to the Internet. And specially-made tablets would cost on the order of tens of thousands of dollars each (they'd have to be specially designed and manufactured -- and with these extra costs only being divided among a small number of tablets, the costs would be extreme. Think $30,000 hammer)
If they use retail tablets, then mischief can ensue pretty quickly. Remember that prisoners have a LOT of time. And remember that, while most of them are dumb as a box of rocks, some are pretty smart...and would be able to exchange those smarts for quite a bit of value.
They’re ordering a batch. They can say “don’t put a wifi chip into it” and it will be cheaper because no money paid to the wifi chip maker.