I think you're supposed to tip off the menu price. If you want to eat in fancy restaurants, should you not at least be willing to fairly compensate the servers, especially since you're already getting a major bargain, as so many here have pointed out?
If your meal for two comes to $100 that would be say an $18 tip. If you allot $50 of your meal plan to that dinner (for two people) then the tip would be $9. The server is getting short-changed. If I were them, I wouldn't want to serve DDP users. Before this new tip policy, the servers must have really been getting short-changed, because there's no way the plan allowed for them getting a tip off the actual menu prices.