Because, in previous years, I believe there were a few incidents involving cosplayers and photographers. Mainly there were reports of photographers who kept pestering for pics when the cosplayer was doing something else (resting, eating etc). And photographers who take photos in inconvenient places, causing blocked passages.
Also got photographers who bend very low to take photos which resulted in 'expose' shots especially with cosplayers in short skirts :nono:
Hence, cosplayers tend to be more wary of photographers nowadays and sometimes cannot blame them really...