Can a restaurant add a gratuity to a check for only two diners?

-Is it legal for a restaurant to automatically add the gratuity to the check if there are only two diners?As long as they advertise a service change - they can. Back in the day of counter service, there used to be a minimum charge of 50 cents per person, even if only one person was in a group.
Yes, I have experienced it, and it makes me angry.
Yes and it was probably in very fine print on the menu. That makes it legal.