I can see this getting more difficult.
I know I’ve written before that I don’t see the point in taking pictures of people I don’t know or who are not doing something interesting, but this is different. What got me thinking was an article on Photofocus arguing that taking a picture of someone against their wishes was assault. It could be, but it also gives the power of claiming assault to anyone photographed in public. There was a recent case of someone apparently photographing women breastfeeding in public. The photographer may be a creep, there is not enough context to tell, but it shows a desire for legislation.
On the matter of context, I think that depends on the subject of the picture. If I’m taking a picture of a landscape (yawn) or a building and there are people in the frame, they are usually there as part of the scene (or because I couldn’t get a picture without people in it). The point of the picture is not the individual people: any people would do. This isn’t assault and I shouldn’t need their permission.
If I take pictures of people engaged in a sport or activity then, providing I am allowed to be there, I don’t need their permission. Their activity and skill is the point of the picture.

Even with individual people I think there is a difference depending on whether the person is identifiable or not. But this is also where it starts to get difficult, and it’s down to intent.
Different countries’ laws vary on public privacy. Some, like the UK and USA, have a basic assumption that whatever is visible in public is not private. Hence the argument over the breastfeeding women. It may be creepy but it’s legal.
Why is any of this important? Because it could become more difficult. If any new laws are passed that set limits on what we can photograph they are almost certain to be badly phrased and to restrict previous freedoms. I’m also worried that new laws get made from extreme cases, so end up as bad laws by that route. A ban on photographing people without permission is unworkable. A law that says anyone objecting to being photographed must have their picture deleted is equally unworkable and leads to threats and violence. I’ve seen it lead to threats even without legislation.
What do we do then? Perhaps if we are close enough to the subject for there to be interaction, we just ask if it’s ok? Perhaps it is just a case of being polite?
I’m not saying we should be scared, but I do think we should be mindful. I really don’t want to be confined to landscapes.
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Updates – Apple now police your pictures, and legislation of some form is coming. Not that I take creepy pictures or even think people should, but I do worry what the result of these new rules will be.