That really has nothing to do with Hotwire, it's just how it is.

Chris Elliott in one of his more stupid pieces a few years ago (I know, it's hard to rank them in order sometimes) says to avoid the fee you should park the car outside the rental return garage, put the keys in the glove box, and lock the car. When your rental is actually due to be returned, call your auto club, and tell them you locked the keys in the car, but had to catch your flight and ask them to tow it to the rental agency.

It's common to reprice a rental for an early return if they think you rented for a longer period to get a lower rate (weekend rate but only want it Friday, or reserve for a week to get the lower rate). It's just plain silly to not take it the night before when you tell them it's because you just want to jump on the shuttle in the morning.