Glenn Reynolds has a Popular Mechanics piece on the prevalence of police, security guards, and other officials who falsely think taking pictures of them is a crime. An excerpt:
Here’s how bad it has gotten: Not long ago, an Amtrak representative did an interview with local TV station Fox 5 in Washington, D.C.’s Union Station to explain that you don’t need a permit to take pictures there–only to be approached by a security guard who ordered them to stop filming without a permit.
The general rule is that photography is permitted in public spaces. Further, when law enforcement officers are in a public setting, they should have no expectation of privacy. For further reading, this subject is covered in detail by Carlos Miller’s excellent Photography is Not a Crime blog.