There is no reasonable expectation of privacy when in public. While it's certainly polite not to take photos of strangers and post them without asking if it's ok, you shouldn't morally or legally need permission. They're in public. Their actions can be seen by anyone else in public who crosses paths with them, whether online or off. There are cameras everywhere, and a random person with their smartphone should not be held to a higher standard than a surveillance company or security camera. I recently had someone give me a hard time because I had posted a selfie where someone else was in the background smoking a cigarette. The person was recognizable but far from being the focus of the photo. It was a complete coincidence that they were in the background, but a friend of theirs recognized them was angry that I had posted a photo of them without consent doing something that they didn't want shared on social media. Well, I'm sorry, but if you don't want people to see you doing something, don't do it in public. I chose to immediately remove the photo out of politeness and to avoid drama, but in my opinion, people have no right to be upset if a photo of them doing something in public goes viral.