While there may be a reason they need to take a hard line on people asking for preferred parking spaces for reasons other than a disabled placard, I doubt that this has much to do with alleviating any parking issues from a guest perspective. The more they raise the cost of parking, the more they push people to the Signature AP, and then those people come more often and park more often and park for shorter periods, leaving random empty spaces that Disney can't refill. Charging more to people who are likely to stay longer (because they paid more), or pay more because they're staying longer seems like a win for them.