Had you driven that Prius so that the tank was empty, you might have gotten 40 mpg at the pump instead of 48 mpg (or whatever). You might have blamed it on traffic, or the car, or the fact you were in a hurry... but actually you would have just bought a bunch of extra fuel.

Where the car was half full, it's less of an issue for the next renter because it's visible... but plenty of cars can do 60 miles or more with the gauge showing 8/8. In those circumstances, someone who returns the car when it is genuinely full will subsidize someone who returns it with the needle primed to drop.