This is pretty much a poster child for a good picketing. They absolutely should do it. Hand out literature at the picket describing what occurred. I guarantee you will cost them far more money in lost business than they would have lost in the mandatory tip. I believe there are even organizations that have will have volunteers picket for good causes. They should seriously do that.
The Nom Nom Cat
Couple Arrested for Not Paying Tip
Happy Lil' Elk Frolics in a Pond