It makes sense to me. In many cases, you're not doing them a favor by returning the car early. Not only do they sometimes not have space for it, but they also didn't take advance reservations to rent it for that day since they had planned on having it rented to you. It's a double whammy for the rental car company if they're short on space.

Just imagine what a busy location would look like if everyone scheduled to return on Saturday suddenly decided to bring their cars back on Friday. Sometimes it's necessary to change plans, but I can understand why they'd want to discourage this practice with their pricing model.