But it's the incestuous relationship with Disney I'm talking about...and here's hoping talk of Disney and incest never extends beyond this.
You brought up the spas. I honestly don't know how the spa at Saratoga operates, but my guess is WDW "rents" the space for the spa from DVC for pretty much nothing, and it's all numbers on a spreadsheet so it's not like money changes hands. And in exchange for the sweet rental DVC members get discounts on services, so goody on us. But that's how Disney profits from DVC without DVC members necessarily sharing in those profits. Similar deals with any restaurant or lounge services in the DVC resorts - DVC has a contract with WDW where DVC rents out the space, WDW uses the space for kitchens and dining areas and shops and bars etc, DVC in theory isn't exposed to the "risk" of running their own restaurants and bars and gets a flat fee. WDW assumes the "risk" but reaps the profits.
But the roads are different. Paying for parking would in theory have to go back into DVC's account, and the profits would then have to be shared back with DVC members in the form of stabilized or reduced maintenance fees, which could become problematic. The only way to get around that is to argue DVC does not own the property where the lots reside.
But it's also possible you're right, DVC DOESN'T own the lots attached to the resorts, and at any point they can start charging DVC members for parking. Or they choose to pit DVC members who drive against those who don't "sure, you're paying to park, but now everyone gets a rebate, why should people who take Magical Express to WDW have to pay to maintain the lots they never use?" But I think then the argument would be "well, the DME and theme park buses DO use the roads and lots so members who don't drive DO assumed some responsibility for them. And if I never use the pool or the gym or the community hall, why should my maintenance fees go to THOSE services, I demand a rebate!" I think it's behooves WDW AND DVC AND its members not to get into that kettle of fish by saying "DVC members still get to park free, it's a 'perk,' y'all!"