I do kind of agree, but I have not much hope for a free market where a company wins by being that much better than the rest, where they could just go with the flow and charge the same fees as anyone else.
With gas cars, there's a clear cost to having an employee go out and refuel it if its not returned full. This is much less justifiable if the company can just charge the cars on-site, either quickly or slowly.
A compromise could be that if a car is returned with less than x%, where it must be charged before going out to the next customer, then charge more but if its returned with 80-100%, don't charge extra because it could go out to the next customer easily. I've rented plenty of cars without a full tank.