PSA for all folks new to America. Tipping is the expected custom here. Not tipping when the service was adequate or better is considered rude and those that don't tip considered self centered jack asses, morons or both. If you think tipping is stupid we can provide you a list of countries where it isn't the norm and you can head over there.
The restaurant was completely in the right this time, however the time they added the 18% without saying anything they were completely in the wrong. They gave her the terms up front and she didn't like them case closed.
Lets be honest here about the petition too, 300 people signed it? I bet no more than 2-4 of those people would have ever gone to this restaurant in any case.