Look at ot from a business standpoint. A group of 4 comes up, maybe spend 20 bucks total. Maybe only one person gets a drink. OR they can charge a fixed price for a meal and require everyone to pay that price to come up. The lounge is not self sufficient, it relies on sales of food and drink. Therefore if not everyone is buying when they come they are losing money. Disney will squeeze every penny they can for such prime real estate. Doesn't matter whether it is DVC or not. The bottom line is that the rooftop lounge is an extra that can go away at any time. And if it is not as profitable as it could be rest assured Disney will find a way to make it profitable.
Disney did look at it from a business standpoint and tried it with Reunion Station. That model lasted 8 months of most likely being in the red the whole time, while Top of the World has been open for almost 10 years.
Could their system use a tweak in order to maximize profits? Sure. But rewriting the whole business plan to restrict it more would not benefit them.