Remember the good old days when car rentals, at least from the major companies, included gas. If you bought gas, you handed over the receipts when you returned the car and the cost of the gas you bought was deducted from your rental. You didn't know, then, how much they were charging you for a gallon of gas, but when a full tank of gas was only $5 or so, the convenience of not having to fill the tank before returning was worth it. Of course back in those days, there was no "unlimited" mileage, so you paid for gas by the mile. I like how they're doing it today - buy your own gas and drive as much as you want.

I've rented from Thrifty in some locations where they want to see a receipt for a gas purchase if the car was driven less than 50 miles. I suppose it's defense against people who don't drive far enough to move the gas gauge off Full, so they don't replace the gas they used. There was a story going around that Hertz, as part of the return process, was interrogating the car's computer to see when gas was last put in the tank.

For what it's worth, only once was I ever asked to show a gas receipt, and when I showed it, the agent said "Good, you read the instructions."