When we were there in 2016, you had to be a DVC member staying on points. If you were a DVC member staying on a cash reservation, you'd be charged for parking. Similarly, if you were not a DVC member but staying on someone else's points (i.e. renting), then you will not have to pay for parking.
I'm not sure if this is still current, but at the time, the front desk explained this is how its coded in the system so that when you insert your card in the parking garage, the system recognizes the points reservation and you are not charged for parking.