How do you know they don't top it off? At the local HLE, every single care returned gets topped off, if it takes over some amount of gas they charge the renter, if it doesn't it's full for the next guy.
How do you know the person before you doesn't just put gas into the car until the needle hits F without filling it? I know others here disagree, but I believe it's pretty easy to get the needle to F without putting in that last gallon or two if you really wanted to. Unless you fill the car as soon as you leave the facility once you pick it up, you really don't have any way to know if it's full or not.