This policy actually started some time ago (a couple years, IIRC) with Avis. AFAIK, it is now more or less standard policy at Avis, Budget, Dollar, and Thrifty.
From what I've heard, if you can later prove that you did fill up (such as providing a screenshot of the credit card charge on your account), all four customer service departments of all of the above brands will be happy to remove it. Oftentimes they won't even require the proof; just a quick note to them will be enough. Try this:
http://www.thrifty.com/CustomerCare/...xperience.aspx