The practice may not be "formally" prohibited, but I imagine there are any number of things at WDW one is not supposed to do, even if not explicitly stated.
Disney charges for parking at the theme parks, does not charge for parking at Disney Springs (common for any shopping centre), and does not provide transportation from Disney Springs to the theme parks.
It's pretty obvious that they don't intend for people to park there when visiting theme parks. Yes, people can, but the obvious risk is that parking rules will change in the future if too many people do it.
Not judging anyone, we all bend certain rules from time to time, I just find it silly to argue that this is appropriate use of the DS lots.