It's not an ordinary McDonald's; it's the McDonald's in Paris next to the Arc de Triomphe. Given its location next to one of Paris's largest tourist attractions, it probably serves a much larger number of customers per day than most other McDonald's locations, and it's a potential target for anti-American sentiment. These could both be reasons for enhanced security, although I'm not sure why they'd mind cameras. (OTOH, I'm also not sure why they'd mind cameras if it were a mafia front, since presumably all of the illegal things happen behind closed doors.)