Cus The nets have to pay for all this so they prob say so
At Metlife and Yankee stadium, they are paying the same thing and are not limited to what you can fill up on when you go get food.
The company running the food for the building is most likely ordering a set amount of meals per game. Abagail's will get money regardless of what food goes. So unless the company is under ordering and hoping less food goes/people take, I'm not sure why they would have that rule.