Better Leave a Tip

Here's an interesting story... what happens if you don't like the service at a restaurant? Your choice is to reflect your displeasure with a low tip just as you would reward excellent service with a large tip. Well here's an interesting story from New York where someone was arrested for not leaving a large enough tip. And I've always wondered why large parties are told what their tip rate is going to be. I know many times when I've eaten out, the service is perhaps worse when with a large group, so why is it fair to force the patron to pay a high tip? After all, aren't tips supposed to reward good service?