Glenn Reynolds writes in Popular Mechanics about The War on Photography in Public Places:
What should you do if you're taking photos and a security guard or police officer approaches you and tells you to stop? First, be polite. Security people have tough jobs and probably mean well. Ask them what legal authority they have to make you stop. (If you're in a public place, like a street, a park, etc., they have none; if you're in a private place, such as a shopping mall, they may have a basis for banning pictures.) Krages advises those hassled by security guards to threaten to call law enforcement. If it's an actual police officer who's telling you to stop shooting, ask to speak to a superior. And remember–you never have a legal duty to delete pictures you've taken.
More importantly, we need better education among security guards and law enforcement. In Britain, the country's police chiefs' association is attempting to educate officers about the rights of photographers. So far, nothing like that has happened in the U.S., but it should. Trying to block photography in public places is not only heavy-handed and wrong but, thanks to technology, basically useless. With the proliferation of cameras in just about every device we carry, digital photography has become too ubiquitous to stop. Let's have a truce in the war on photography and set our sights on the real bad guys. Who, it seems, don't carry cameras anyway.