Actually the deal, typically, is that if you buy a full tank up front, the per-gallon price is at par or even a bit cheaper than local gas stations, so you save money and gain convenience (that's the pitch). By contrast, if you don't buy gas up front, and don't return the car full, that's when they charge you an enormous per-gallon price. That price is highly profitable and also makes the "up-front" option seem more appealing.
(Just take 10 minutes and return the car full, and don't worry about any of this)
(Just take 10 minutes and return the car full, and don't worry about any of this)