This professor suggests anti-gun activists should not pay their bill in a restaurant if they see a gun:
Professor Jack Russel Weinstein advocates theft as a legitimate response to 'fear.'
Let us examine the potential liability of the restaurateur. The anti-gun persons have, presumably, voluntarily given up their right to keep and bear arms. Thus the restaurateur cannot be liable to these individuals. However, if the Constitution of the United States of America was actually ever used in this situation, the pro-gun person could hold the restaurateur responsible, if the restaurateur were one of these people who ban guns on their premises.
The right to keep and bear arms and the right to self defense may conceivably be trumped by a private property owner's right to have things done as he wishes on his property. However, it only makes sense that, should one require one's customers to deprive themselves of self defense, that the property owner should take steps to provide a defense of the establishment and its customers in lieu of the customers being able to defend themselves properly.
In other words, the gun free zones are liable, not the establishments that allow customers to carry.
Hat tip to NoAgenda. It is worth noting, John C. Dvorack brought up the point that if some idiot leaves a printed card at a restaurant saying they left because of 'fear' then they didn't leave because of fear, but premeditation. Theft cannot be sacralized via protest, regardless of what progressive may think about the matter.