I think a very, very small % of people will actually benefit from this as you pointed out, instead of paying $7/gal x 5 gallons they might just pay the smaller charge...

but any rental company that charges customers for forgetting to get a receipt is bascially inferring the customer is lying, and the customers won't be very happy with that.

MANY times as soon as I've driven off a rental lot I've watched the car go from full to 7/8ths, meaning the renter before me didn't fill up, OR worse yet the renter before me was charged the fuel fee but the agency didn't fill it up.