It is part of the general paranoia. The law says you can photograph an public thing, whether it is a building, a bridge, the Liberty bell, anything so long as you do not trespass on private property without permission or invade someone's privacy. Unfortunately too many security guards either don't know the law or don't care. But, it is better to leave the camera in your bag than get beat up by a guard. On the other hand if you are just walking by and they try to hassle you for having a camera in your possession, call the police and then sue the $#%@ out of the guards and their employer. You can start with unreasonable search and seizure, and unlawful detainment. If they take the camera or film you have them for theft/robbery too.