I believe it must be the Promoter / Director's responsibility to have signage at the entry points, stating "No photography of the artist's work, without the express permission of the artist".
A) For the artist to have to display such signage in their own booth, takes up precious real estate, we need for our displays.
B) Some artists might not mind the pictures. Fine, they can give permission.
C) For artists who think this is unnecessary... if your medium was photography, you might feel differently. Photography can be duplicated semi-reasonably by a third party. Especially if quality is not an issue.
D) Without such signage, we are faced with having to say "no" or police the customers. Easier and more pleasant to say "yes" than "no". Neither stopping someone, saying "no" or policing, is favorable or conductive to success.
E) Proactive is far better than reactive.
F) As a photographer the signage would be cumbersome, not simple. "No photos" - "No photography" - a camera with the not sign across it - are all not appropriate as they confuse the customer, being our art is photography. Therefore we would have to put up more wordage to be specific, not confusing. Thus larger signage.
It does not matter about copyright, common sense, or intellectual property rights. We are there to display and sell. Not to argue, educate, debate and police the customers. Why should WE be the bad guys saying "NO"? Why not let the promoter take the burden off our shoulders?