WDW's business model up to this point has been based on the notion of "keep them on site." Tier the park tickets so you have the illusion of value the longer you stay (or as I think they put it "the longer you stay the more you SAVE!"). Have a bus pick you up and drop you off at the airport. Dining Plans so you don't go offsite to eat. Even shops with limited convenience store items so you don't go offsite to buy a half gallon of milk or bread or cereal if you want to eat some meals in your room.
As a result one of the reasons they could charge as much as they do for rooms is because "all fees are included."
Now, the idea for charging for parking might be "if you don't have a car you have less incentive to go off property" and that helps keep you on property.
Another idea for charging for parking might be to help offset an increase across the board in room rates. I know others have argued "just increase the room rates!" But if you don't drive, and charging someone who drives to WDW for the privilege of parking means their rates go up but your rates don't? You might be thrilled with the charge for parking.
But whatever the "industry standard" might be, I think adding additional fees is the sort of thing that would give WDW a PR black eye. Because the diehard Disney addicts, the ones who might opt to drive because it saves them the money to afford to go back a little more often? The ones who live far enough away that they can't just drive there and home in the course of a day, but not so far away that flying makes sense (thinking near-but-not-too-near states like South Carolina or Tennessee)? All you're potentially doing is shifting their budgets away from other things like dining or souvenirs. Granted, by making them pay resort fees or parking fees you're more or less making them pay for "nothing,*" whereas paying for food and souvenirs you need to provide a tangible thing and the costs incurred in making and storing and preparing the thing. (*I know you're paying for something with those fees but it's rarely tangible, like parking in theory pays for the upkeep of the lot, the security, etc, nothing you bring home or stuff in your piehole). But if making someone spend $50 in parking means they don't buy a $50 sweatshirt that they wear around back home and remind people about how much fun it is to go to WDW, basically free advertising, will that be a good thing, ultimately? Can they guarantee people will pay the additional fees AND spend as much on tangibles as they would have were there no additional fees, or will this wind up being a rob-Peter-to-pay-Paul scenario? My guess is it'll wind up being the latter, they won't make the additional revenue necessary to justify the ill will.