The question is - what were the terms of your rental agreement? If you signed anything agreeing to "their" terms then your question is a moot point. I'm going to side with the rental company as I'm sure they have everything in writing to protect their property and associated damage from renters.
So many people drive these things away without any further consideration or knowledge of how they work. Then when they screw it up they expect leniency on the repair costs. Buy one for yourself and see how much it really takes to keep it in good condition. That includes driving it.
