The tip in question was really a service charge for serving larger groups. While leaving a tip is not mandatory, businesses have a lot more leeway in adding on extra fees and whatnot, especially if it is already outlined somewhere (e.g. on the menu).
That being said, making it a mandatory charge usually means that it's separate from being considered a tip for tax purposes.
Apparently there have been high level court cases about this (last discussion had a guy posting the case). The 18% extra if portrayed as a "automatic gratuity" is not technically legally required.
It was some case about people getting arrested for theft for refusing to pay.
That being said, making it a mandatory charge usually means that it's separate from being considered a tip for tax purposes.