Might be a culture difference with europe but I find it rude if someone would take a picture of me without asking. I can think of few purposes (stalking, facial recognition training or tracking, sharing in a chat group to make fun of) that you can do with a picture of a random person on the street that you'd not get permission for when you're required to ask

It's always a balance: if someone wants to do it for legal reasons (I just stole their purse and am running away) that's very different. There's almost no law that works absolutely anyway, there can very often be overriding reasons that are already defined in the law (or another law) or that a judge will accept. Just talking about the default case