The parking fee for overnight guests isn't about being fair. It's about Disney doing all they can to make the most money off guests once they enter Disney borders.
Resort rooms themselves are finite in revenue. There is a max amount that they will make off any room at any time. Then they hope resort guests will order room service, use pay per use amenities, eat at the restaurant, etc. BUT, the nature of WDW is such that a large majority of guests staying in them spend very little time in their resort. They're out trying to hit the parks most of the time.
Disney then has this great opportunity to increase resort revenue by bringing in paying day guests. They don't care if the day guests park free. They want them to come spend money, to boost the profit for each resort. If day guests have to pay at each resort they visit, they may very well visit less. WDW doesn't want that.
Returning overnight guests, on the other hand, are less likely to opt out of staying on property over a parking fee. Yes, they'll complain, just as they would complain over an across the board room price hike, but they will still overwhelmingly decide to keep staying on property because that's how they prefer to vacation at WDW. BUT, this may convince some coming by rail or air NOT to get a rental, and that's where Disney makes the money. If they wanted to make an extra $19 a day on Value Resort guests, they could have increased the room price for everyone. But what they want is to make you think twice about that car rental. Why spend money on a car that you can spend at WDW? And if you don't have a car, you'll be less likely to leave over and over again. Sure, you may pay a taxi, Uber or Lyft driver to go out once or twice, but you're still less likely to repeatedly leave property if you don't have a convenient rental car sitting in the lot waiting for you to use it. That's ultimately what Disney wants...for all of us to just stay there on Disney property, spending our money.
The only link between this charge and daytime guest parking at all is that if you have a car sitting in the lot all day while you're in the parks, it's taking up space the paying daytime guests may use. Yes, some people are cheating. They say they're there to rent a boat, grab a meal, etc., and then they just park their car and go to the park. But many do spend money at the resort, and Disney isn't all that overly interested (yet) in bothering to weed out the cheaters from the spenders. Because, hey, that cuts into their profits.
So we can be unhappy about the unfairness of it all, but ultimately if we want to stay and we want a car, we'll pay.
