People get stabbed if they talk to assholes in public. Online we can tell assholes they're being assholes without dying.
Would I like the freedom to express surprise or concern, to help someone or tell someone to use the other door in public? Yeah, sure. But it's potentially a stabbin'.
I overheard an old lady asking a guy for directions somewhere the other day and I spoke up because I knew. But the risk is there. I lived near NYC for a bit: you keep moving, you get off the street. Maybe not even for an old lady lost.
It's not so much a cultural thing as it is a stabbing thing. And if someone's a dick in real life, I think they're more likely to get all stabby if you tell 'em so.
I left the army a ways back, and I don't have the body or the homies to be so confident these days. So I'll let the real-world dicks be dicks and whinge about it from the safety of not bein' stabbed.