In WDW's early years if I remember right all the seven seas hotels & campground was behind the 'pay wall'. If you had to go through the main gate (or side gate by the campground), a 'day guest' at a resort paid the same price as a guest that parked in general parking.
The current infrastructure of the resort is much more complicated. If a guest who's staying at the Gaylord or at one of the Bonnet Creek resorts wants to scout out a hotel to see if it is worth staying at their next visit and you try to charge them for the privilege, they may very well say 'thanks but no thanks' and keep a large portion of their vacation dollar outside the bubble... (but that goes with 'picking up pennies while dollars fly over head')... A guest that stays at a high end hotel outside the bubble is a guest you WANT to check out your hotel...
And think of the Boardwalk. Combine a resort area that has a couple of clubs, some minor attractions, eateries that refuse ADRs (one of them tied to a franchise desperate for good publicity), minimum parking AND a walkway to a couple of theme parks that charge parking, and then don't provide direct bus service to your other resorts.... You design it for spontaneous business, then refuse spontaneous guests....
I