I think it very much depends on the circumstances. If people are sitting down and are in an environment where they feel safe, you can walk over with a friendly

I think it very much depends on the circumstances. If people are sitting down and are in an environment where they feel safe, you can walk over with a friendly smile, chat a little, show them the camera and btw, I took some pictures of you that I hope will turn out really nice. I hope you don’t mind, I’ll be happy to send you a copy. Few people say no. On he other hand, people who are in a hurry or feel vulnerable mostly say no just by default. Does it reflect their true feelings about the matter if they had time to reflect? Hard to know. In the US the situation should be simple. The first amendment must trump any right to any expectation of privacy in a public space.