You're assuming that the cost of having a car sit idle is zero -- which I'm not sure it is. Depending on the station, there might be significant costs associated with having nearly the entire local fleet sit idle -- think of all the satellite parking lots you see at some airports, presumably required for overflow. Moving cars to and from that lot en masse would mean extra staffing.
Meanwhile, as long as they're doing some business, they need staff on hand to clean and prep cars (at least what very minimal cleaning and prepping they actually still do). Having staff sit idle is a waste of resources just as the cars sitting idle is.
It does surprise me that even the cheapest rates rarely impose any mileage limits.