I ran into this situation directly in two different ways.
First situation, friends of mine, at a show in Toronto, chasing off photographers. These same people had been around the year before at the same show (it runs every year) , photographing their wares, and six weeks later guess what shows up in Toronto - cheap copies, low quality knock offs, made overseas, flooding the market. Sure they complained, but there is so much forgery of brand name products showing up at flea markets and trade shows, nobody form customs to the police can keep up with it.
So, when they showed up again, that's why they physically chased them away. My friend told me he would of punched the guy in the face had he the chance. He then would of loved to call the police himself because it might of been the only time to catch the guy and have him charged with counterfeiting. That might sound harsh, but apparently the other guy (the forger, with the camera) knew that too and got the heck out of there in a hurry.
Second situation - this past February, the annual art fair and show in Key West. Been there a few times, some really nice stuff. Took many photographs this year, and years past. Never an issue. Why? First off, I never, ever, ever take photography up close. Secondly, I only walk around with a prime wide angle lens, never a zoom, and I only take general crowd shots.
IMO, it is all common sense. Also in Key West, I walk around a whole day, visiting the different private art galleries with my camera, never an issue. Why, i keep my camera slung behind my back. They all have big signs in those private stores - no photography, so I don't. About the most that ever comes is "sure, you can take a photo photo of the front of the store on the street" or maybe " hey, that's a nice camera." Never an issue.
Also down in Key West, and maybe a bit off topic, but if you ever visit Fort Zachary Taylor state park, and see the old civil war fortress, you will see that a big, active military base right next door. Litterally next door. You see big, big signs, no photographing the military. Nada. I stand there, camera slung, nod hello at some of the military people who look at me and make eye contact, and i almost always get a polite nod back. Then some idiot stands beside me, points their little compact or smart phone or even iPad (the number of people who walk around Key West with iPads shooting photos is sometimes mind boggling), and boy oh boy, do they get a dressing down.
So to me, this whole issue boils down to common sense. Let me go back to the Key West art fair. A number of those people/artists selling products were photographers selling prints. Same for some of the private art galleries in Key West. Talked to several of them, the art fair and the galleries. All of them have "no photos" signs or something similar. All of them I talked to in one way or another have stories about people photographing their work, and then using it on a web site or selling a copy cat forgery or something similar.
To me, it all boils down to respect for fellow photographers, nothing more.