Referee's, like players, get better as they officiate (play) better players. NHL Referee's referee the BEST players in the world, and as such, they are best able to cope with the high class international tournaments. Just like the best soccer officials come from Europe (they get to referee the best on a regula basis), so the best Hockey officals come from NA.
BTW, the lockout and resulting influx of talent to Europe will only make European referees better. I suspect that many European referees are better (or certainly capable of becoming better) than many in the NHL (and epecially the AHL and lower leagues). I think the NHL would be well served to recruit referees from around the world, and not just in the AHL.