Along the same lines are the “Americans forget that other countries exist” comments.
Does American culture have a problem with ethnocentrism? Generally yes. Is reddit an American website where the vast majority of users, comments, posts, and topics are American? Also yes. When I’d see one of the “you’re an arrogant asshole for assuming this post is about the US” comments, I just assumed it’s someone being pedantic and contrarian for the sake of it. It just ruins what is sometimes a valid point.