Many restaurants have written in their menus that a mandatory gratuity of a specified amount [in the city I've most commonly seen 18%] added to the bill for parties of certain sizes [smallest I recall is 6]. It's usually clear in the menu.

They can't force you to stay. You can leave and eat elsewhere if the mandatory tip bothers you.

I'm not a judge or a lawyer. But, I don't know if such a accessing a mandatory tip in this manner is legally binding.

But, there are two sides. Obviously there is a history of larger parties screwing wait staff members or they wouldn't feel the need to make the rule. Certainly, if a tip is "guaranteed," the level of service is likely to slip since incentive of trying harder for a better tip is gone.

In any case, having everyone know the service at your place SUCKS by having these people arrested and their story all over the news cannot be good for business.