Wouldn't it be more likely that, since the majority of LessWrong users are from the US, most posts are US-centric because that's what the poster himself is familiar with?

I mean, certainly we could pose a line of reasoning to create a post hoc justification for the practice, but what seems more likely is that US-centric posts are reflective of poster, not audience, knowledge. Unless you think we'd have reason to suppose that posters would readily be plucking examples, etc. from their immense knowledge of the UK or Nepal.