What is "Fair Use?"

I went to an art show a couple of weeks ago and many of the artists and craftspeople had signs reading "No Photography." I guess they were either ignorant or hoping that fair goers were. How can someone demand that when they are in public and there is no expectation of privacy? It was really ironic because some of the artists either photographed or painted things that other people had made, I.e. buildings, famous structures, copies of artwork etc. you would think that the rules that people further should apply to them also. Just saying!