Why is there sometimes a drop-off charge for one way rentals?
Each rental station owns their cars. When a car is left in another city, the owning station needs to get the car returned back to them, either by holding that car idle while waiting for a rental which will return it to its origination point, hiring someone to drive the car back or by having the car shipped back to them. In all cases there is added expense to the rental car company for such rentals.