Hotels & Restaurants in Florida (including all those on Disney property) are defined under state law as "Public Lodgings" - the operative word here being Public.
An operator of a "Public Lodging" may deny or revoke access to someone for a variety of reasons, including disorderly conduct, public intoxication, intent to distrubute illegal substances, etc, etc. Absent one of those obvious common sense reasons, public access means public access.
Disney certainly has the right to try to discourage parking by day guests, but the "rule" referred to in this thread has little or no legal standing. As for being towed, that's not going to happen. Unless signs are prominently and frequently erected announcing "Parking is for Resort Guests Only. All others will be Towed", no basis for towing exists. The "Rule" doesn't count.
Disney DOES NOT TOW. They have no way of knowing where you are, or even if you still in the hotel, and they are not going to tow someone's car that is shopping or eating in one of their hotels. They are not going to waste the man hours on searching for cars in their lot that are not supposed to be there. It just not going to happen. Its just bad for business. They want people to come to their hotels to shop, eat, look around because not only will they make money, they may also encourage that person to stay at that hotel the next time. Once again, i have been parking at Disney hotels for over 20 years and have never had a problem. Never even seen a car get towed.