The Oxymoron of a Mandatory Gratuity

So lately The Husband has been quite annoyed with how gratuities are now expected when we are out and about.

Depending on the level of service, both The Husband and I will usually leave between 15% and 20%, but what gets us is that if you leave less because of the level of service you are considered stingy.

The Husband is currently working on booking his Christmas party for work and while doing so, because the group is more than 8 people, is finding that most places are informing him that there will be an automatic 18% gratuity added to the bill.

Just for reference, here are some definitions as per Merriam-Webster:

MANDATORY

1: required by a law or rule : obligatory
2: of, by, relating to, or holding a League of Nations mandate

GRATUITY

something given voluntarily or beyond obligation usually for some service

I do believe that by the definitions above, adding a automatic gratuity then makes it mandatory, which in my opinion, completely negates the purpose of a gratuity. To me, it should be optional to represent my level of satisfaction of the service I received. My question is if the gratuity is automatic, but the service is atrocious, do I have an option not to pay it?