I did a small amount of research into this question. First, consider what happens when you enter a restaurant: You sit down at the table and order. By placing an order you (legally speaking) enter into a contract with the restaurant: “I pay you and you give me these dishes/drinks”. If one party does not hold up their end then (in principle) it is reasonable to say that the contract is broken. If you give me the wrong dish I don’t have to pay for it, If I dine and dash I can’t come back and try to start a new contract.
Same deal when you enter an all-you-can-eat joint: I pay you X and I can scoop slop from the trough onto my plate (Larry David’s phrasing, not mine). I’m sure contract law varies from state to state, but generally speaking for a contract to be valid both parties must enter in good faith. Going in to an all you can eat joint and staying all day is not in good faith - how would you feel if the buffet stopped refilling the food? Or someone at the next table put food on his plate and threw it out?
So by going and staying all day you are entering the contract in bad faith. These sorts of things didn’t have to be said years ago because we lived in a society with rules. Now they have to be spelled out clause by clause because many people don’t understand that there reasonable expectations that people have of each other when entering a contract.