I have been hearing a lot lately about restaurants charging a "mandatory gratuity" on parties of five or more people. I personally don't think that this is right.
A gratuity is defined as "something given voluntarily or beyond obligation usually for some service" (as defined by Webster's Dictionary) and the term mandatory contradicts the terms voluntary and beyond obligation. I understand why some restaurants want to do this, I have worked as a server before and have gotten stiffed on a bill for a large group of people. A gratuity is for exceptional service, and what happens when a group of people get rotten service?
I have heard of one such situation where the group refused to pay the "gratuity" and was actually locked into the restaurant until the police were called in.
Personally it's a gift of money, you cannot expect me to give a gift when I am told what to give and how much to give; that is a demand.