When you rent a gas car, you don't have to bring it back full. You have to bring it back with as much, if not more fuel than it left with. So if you get it with 1/2 a tank, you can bring it back with 1/2 a tank. They usually give you a chart when they give you the keys that tells you how much fuel it has currently, and how much fuel you would need to add from empty to avoid paying extra fees.

So whether you're in electric or gas, obviously nobody is going to expect 100.0% fuel capacity on return. This is customer arm flailing, being pedantic and dramatic about something that is very clear. "Maybe if I make a bunch of noise they'll treat me special!"