.......you're likely spot on with the "captive audience" theory which will somewhat limit accessibility to other area venues
.....I thought about this too, and have somewhat followed (meaning I paid attention here and there as time allowed) the "Mini-Vans" initiation at the parks / resorts. As I recall, the Mini-Vans were rolled out with a certain fee attached ....and then shortly thereafter ...the fee was increased a substantial amount.
.....now ...I'm sure there will be those that cry out "Conspiracy Theory" at me ....but ....had they NOT upped the fee a while back ...and had done so now, or shortly after the parking fee was announced .....you can only imagine the outcry from the Disney faithful. It would make this debacle's affiliated outrage seem like a garden party.....