First, the "policy" point is really more a matter of parking policy than transportation policy.
Parking at DTD is for guests visiting DTD to shop, dine, rent boats, be entertained, or just wonder around. The amount of parking at DTD, especially near the Marketplace section, is limited. It's not meant for all-day parking by people going to the parks. There are signs saying the parking is for DTD only.
The buses have no signs limiting their use only to passengers who qualify by virtue of where they're staying or what kind of tickets they have. So there's no current, published policy (although there was one 25-30 years ago).
The real reason this is a bad suggestion is not because of any official policy, but because it's a huge waste of time. There are no direct buses from DTD to the four major theme parks or TTC. That means having to change buses at a hotel, and that means having to wait for a bus twice, as well as time on the bus, which often involves making multiple stops.
A family trying to use buses to save $14 on parking (or whatever it costs now) will see each member of the family wasting up to an extra hour in the morning and up to an extra hour in the evening. Considering the cost of park admission, why would a family want to reduce their time in the park and spend it waiting at bus stops and traveling on diesel-engine buses, in addition to their time in the car from and to their off-site hotel? On top of that, arriving at a park later means that a 15-minute line for a park attraction might have become a 45-minute line.
