It's an Abuse of Services to display all or part of the Ad Unit to any user located outside the US.
This one threw me a bit. I can understand requesting that audience is primarily US, but how does a YPN publisher guarantee that someone from outside the US doesn't come along and see an Ad? Even if my site were to be about US specific topics only, anyone around the planet might happen to arrive there.
I read here where a non-US webmaster clicked their ad and essentially got blocked from the YPN side of things, but that doesn't seem like a safety net that can be relied upon, as the terms are talking about displays of ads.