I wonder if they're not down to the grisliest part of the task, digging out the huge numbers of bodies that must still be trapped in the debris. That may be the reason for the "no cameras" regulation.
I don't know that legally the mayor can suppress photography at the site, whether it's a crime scene or not. As I recall, numerous photographs of murder victims, burglaries, etc. have appeared in the newspapers over the years. A photograph in no way disturbs the evidence. There's more going on here than meets the eye.
One of the above posters indicated that the police, FD, and recovery workers were being incensed by tourist types making a mockery of the scene. I suspect they appealed to the mayor to take this action. As far as the legality of the action I can't say for sure but I doubt that it is a legal action. Who is going to argue with Rudy about it?