I have actually wondered about the same thing. I had this brilliant photo opportunity..a hairdresser-lady-person was sitting on a bench, with one of them fake plastic heads with long hair sitting next to her, she was looking away, the plastic head was looking straight at me. The whole scene was so absurd and so hilarious, I really wanted to get a photo, but if I'd asked I would have lost the moment, and I didn't really want to just take a photo without her knowing. I read about this in a photography book I was borrowing - apparently it is legally and professionally perfectly OK to just take photos of people without asking first, or without them knowing, but I don't know if it's ethically right. I would have real trouble doing it.