I think the problem with people is that the rules vary from country to country. My understanding in the UK, where I live, is that there is no law against taking pictures of people in public places, unless they are children in care, and then you need permission. Of course, you cannot possibly know whether children in a scene are in care unless you go and ask them! I suspect in a crowd scene there's some lenience. On private land, you're supposed to have permission to photograph people for the purposes of editorial and certainly for commercial use. Some places are privately owned public spaces, and I believe you should still have permission in those places, but I suspect a lot of people don't.

Then there's the ethical issue. A lot of parents think we need permission to photograph their child. I don't think that's true in law, if the child is in a public space, but ethically, it might be considered appropriate to get permission.

Then you have the issue that the internet is global and I don't know how the laws are different in different countries. So the bottom line is, I've grappled with this myself, and tend to take pictures of landscapes and animals more than people. But sometimes people get in the way, then I have to make a judgement call. I think in terms of what's acceptable it may also depend upon how much the person is the subject of the picture. If they're far away and hard to recognise, it's less likely to be an issue. I'm certainly no expert on this subject though.