It's not only a culture difference but a legal one. It's not just rude, it's actually illegal in some parts of Europe. Some parts of Europe remember what it was like when people went around photographing other people and making reports on them. Furthermore, in the Netherlands, for example, there's a portrait right that photograph subjects have.
Well yeah, there is that, too. I don't really care about the law so much as ethics though because law follows societal norms and this law isn't commonly enforced anyway
