I kind of understand the need for a mandatory gratuity for larger parties (even though it is not uncommon for the house to take a cut of the "mandatory gratuity" for large private parties) so sometimes the people who provide the excellent service are not getting the entire tip you are leaving for them. However if the service is substandard or if there was a problem that was not resolved to your satisfaction, I think that an eating establishment should waive at least a portion of it.
Several years ago our family went to Turkey Run Inn at Turkey Run State Park in Indiana for thanksgiving dinner. There was probably 10 of us in our party. The dinner was a self-serve buffet and basically the wait staff was non-existant, so if you wanted a refill on your drink, then you had to once more serve yourself. Imagine my surprise when the check came and they had added a mandatory gratuity because of the size of our party even though we basically served the entire meal to ourselves. We paid the gratuity since we didn't want to make a big deal out of it in front of the whole family, but the next day I called the restaurant manager and explained my objections and that I didn't understand why we were being charged for service we didn't recieve. All she said was, sorry maam, thats our policy, theres nothing I can do about it. As a result, we will never return there again.
My point in mentioning this is this restaurant will lose a lot more from the way they handled this situation than the gratuity this man didn't pay. They will lose lots and lots of business.