Steve, a freelance photographer, was stopped while taking pictures in a San Francisco MUNI station, told that he was breaking a post-9/11 law against photographing San Francisco's public transit. He challenged the MUNI cops to name the law he was breaking, aware that such a law was unconstitutional, and they — unsurprisingly — couldn't identify the law. That is because there is no law. They were lying.
So then they called the real cops, who proceeded to dress Steve down for breaking this nonexistent law — for being a troublemaker who wanted to exercise his constitutional rights and ply his trade — and threaten to trump up a trespassing charge and jail him for the weekend if he didn't meekly acquiesce.
Officer Primiano expressed extreme frustration with me as soon as I began speaking of my rights to photograph in public places. She wanted to debate the wisdom of my taking pictures and asserted that in the wake of the Sept 11th attacks on our country, I should be more interested in aiding officials in their efforts to increase security than my rights as a citizen or journalist. Despite my calm statement of my side of the issue, Officer Primiano waved her hands in the air, stated, "This guy is really pissing me off", and walked away, leaving Officer Ryan to talk to me. Luckily he exhibited a more rational, professional demeanor.
However Officer Ryan was of the opinion that I should not be taking photographs. I explained to him that I didn't want to argue the wisdom of my taking photographs, or the efficacy of a ban on photography in the MUNI System should one exist. All I was concerned with was the legality of my actions. If I had in fact committed a crime by taking photographs, I should (and in fact wanted to) be cited under the relevant law so that I could then pursue the matter in the courts and assert my First Amendment rights. Officer Ryan told me in a very straightforward manner that he did not wish to allow me the opportunity to assert my constitutional rights in court.
After walking over to the group of Fare Inspectors and BART Police Officers, Officer Ryan returned to speak to me. He expressed his frustration at the situation and me by saying: "Would it have been so difficult for you to just stop taking photographs when these guys told you to stop? If you weren't on your soapbox, I'd be out fighting real crime rather than standing around here dealing with you." He expounded further, "Even if there is no law forbidding photography in the MUNI System, the Fare Inspectors have the right to refuse you service for any reason they choose, including taking photographs. Once they refuse you service they can swear out a citizens arrest for trespassing. I, or other officers, will book you and you'll spend the rest of your weekend in jail. It won't be for taking photographs, so your weekend would be ruined yet you'd never get a chance to argue the matter of taking photographs before a judge."
Update: Jen sez, "for photographers who don't know their rights (and there are a bunch of them!)"
Update 2: Tony directs us to this handy primer on California tresspass law, noting, "police would have to convince court of 'purpose of injuring any property or property rights or with the intention of interfering, obstructing, or injuring any lawful business or occupation.'"
Update 3: Niall sez, "here's a pdf guide to photographers' legal rights in the UK."