You got "lucky" in the past. Think about it, if they didn't put a hold on your debit or credit card for the estimated cost of the rental, how would they have any security that the fees would be paid other than your "cross my heart"? Just cause you have room on your card at the time of rental, doesn't mean you're not goin go out and charge a wide screen tv with nothing available on your card by the time you return the car...