I've never heard of a lawsuit (at least covered in Auto Rental News, anyway) against a rental agency due to overinflated tires, so I don't think the liability for traction is on anyone's minds. (That might change if a customer ever wins a lawsuit on that basis.) As for wear, the rental companies generally don't keep the cars long enough for that to be a factor--by the time it's even close to time to replace the tires, the cars are typically long gone from the fleet.