In fact, the presumption of photography at will is so great that at most trade shows and conferences today, as an attendees, you typically give them the right to use your likeness when you register. (Read the fine print.) In fact, if you, for some reason do NOT want your photo taken, you have to assert that point with the show management (if the fine print allows that option - if not you would have to forego attending to ensure you are not videotaped or photographed).