We were having this discussion on facebook earlier.
I have a feeling they do it so that you don't choose to park at the hotel daily to go to the parks. The hotel parking is not extraordinary expansive.. I remember driving around PBH looking for a spot in September one year. I think $20 is a bit much, but they have to charge more than the deck and charge annual pass holders because other wise AP holders would be smart and park at the hotel that's closer. I agree that guests should have a cheaper rate though. Even 5$ off a day adds up. And then someone said... "well make it accessible by room key only." Good idea but...
Room costs would still go up. It's the same battle we have with our rental homes and pool heat. Either charge everyone for it, or charge an additional fee for the people who want/need it. In their case they decided it's better to charge just the people who bring vehicles. It also makes people say "Okay we need to compact all of our stuff into 2 vehicles" rather than it not mattering and them bringing 4. I mean I hate the car fee as much as the next guy, but I just go ahead and figure it into my hotel costs. So instead of my room costing 215$ a night it's 235. That's all they would do if it they added it into the cost.
In my business we do a whole inclusive rate thing because it's the mind game of rentals. Its not that other hotels don't charge you for parking... it's just that you don't see it say "parking fee" you're paying for it in your rate. However, the loews hotels have to show you that you're being charged for it so that, like I said, you don't bring 6 vehicles and clog up their lots.
TL;DR answer. They're never not going to charge parking. There's a reason. It sucks. It's expensive. Budget for it.