Like Suragaha has said and I can agree that there shouldn't be any problem with it unless the person you're taking a photo of decides to tell you they don't want you to take a picture of them, in that case then you should NOT take a picture of them if they say no.
It's common sense to follow this small basic rule in a matter of speaking, but because it's so minor, the mods might not have considered it a major rule but more of a common knowledge rule.