Unfortunately a lot of the car hire companies do this. I've only hired a car from Centauro for 2 days, had to pay for 3 but I did get my fuel refunded when I bought the car back full.
Perhaps they were only offering this service to customers who book the car with Centauro direct.
I've always steered clear of the companies who do this but it's getting harder and harder to do as most seem to be using this ploy to make more money.
Luckily I don't get sucked into this any more as I've bought a car in Spain. 