I'm not sure that that's necssarily a bad thing. Even if it is a bad thing, I'm not sure what exactly, if anything, could be done about USA-only posts.

HN has plenty of USA-only posts all the time, often even Silicon Valley-only posts. Sometimes there are India-only posts. Given a certain critical mass of users living in a certain country, posts which happen to only be relevant to that country can make it to the front page. Particularly when few users will down-vote a post solely for being country-specific.