They are entitled to charge you for their time in going to fill the car up. But you are right. This has become a very common scam to make extra income in the certain knowledge that hirers from other countries will not bother to pursue the matter through the Spanish courts. Happily, Avis Bangkok gives you a full tank to start with, then tells you to bring it back full from the fuel station just around the corner. That's how it always used to be done in Europe.