Good responses. Here's how I see it.

1. Hertz are saying on returns "even if the fuel gauge says 'full' we don't believe that". That's the problem they're trying to solve, and hit customers for.

2. Hertz solution. Show us some evidence you added fuel recently. A receipt from somewhere, sometime recent.

3. So does a Visa receipt mean the tank is now full? Of course not. Depends when I filed up, how I drove, what the car mileage is, fuel price.

The fuel tank gauge is now assumed to be inaccurate. Except on pick up, when 'full' apparently means 'full'.
At that stage of the rental it's a completely trustworthy device.

I'm with Jaimito Cartero. This is a pure revenue grab. The result of a brainstorming session on how to screw extra $ out of the customer.