I think it's fairly common practice nowadays for garages to rely on the honesty of customers to replace the fuel they've used. The alternatives are to charge per mile or supply with a full tank and charge you to fill it on return (always causes arguments). The problem is that you can only put a minimum amount of fuel in so, with a car that does say 60mpg and all you do is take it 2 miles home and bring it 2 miles back, what do you do. After a few people have put none in you're always the guy that gets it empty.