Could be because of .....
1. Parking is very limited at the hotels. Free parking could be an issue.
2. Universal doesn't own the parking lots for the hotels. Not even sure if an outside company other than Loews doesn't manage them.
3. Loews charges for parking at all its hotels as do many of the hotels in Orlando.
4. Having AP holders park for free at the hotels just to take advantage of the closeness of the hotels to the parks (such as HRH) could create a huge problem because of the lack of parking spaces. It is difficult even now to get a spot at the HRH.
5. Keeps prices lower for those who aren't using a car.
6. Makes money for the hotels, although just adding this into the price of the rooms and jacking the prices up $17 for everyone seems to be an easier way to make money.