If you meet people from other cultures, say, while traveling (aside: and you aren't a bigoted idiot), you tend to take some idiosyncracies in stride.
On social media, the default assumption becomes that any weirdly-behaving interlocutor is simply rude.
Conversation
I personally wouldn't say there is such a thing as a common internet culture, so to me this implies some defect in human reasoning.
Do we have a default circuit for appropriate behavior? It wouldn't surprise much, though I don't recall any well-known quality research on that.