Right now it looks like they're requiring Facebook accounts to rent cars, and so they can (presumably) use the Facebook social graph as a rough proxy for trustability. Personally I sorta hate that policy (I've not got a Facebook account...) but it's a fairly good idea, I think.

And, of course many, many said similar things about AirBnB -- "couldn't a thief just book a room for the night and then make off with the TV?" -- and that's proven not to be much of an issue in practice.