If people choose to pay $5.69 instead of returning the car with a full tank, I'd hate to see how much a rental company DOES charge for that.
Although it's been a while, the last time I was there, Budget was getting $6.00 per gallon.
I always fill up my rentals some distance from the airport. Most rentals get about 20 mpg, so about 10 miles away only uses half a gallon... not enough to register below Full on the fuel gauge.
The location of this service station is such that there's not much of a local customer base... just travelers from out of town returning to the airport.