I think it has more to do with the fact that it's so early in the season and non-US gymnasts are typically not prepared enough to challenge the US girls (and nobody likes to "lose" that early in the season). Mustafina did show up one year because the format was different and there was a hefty cash prize involved. It didn't end up going how they were expecting and they weren't all that happy about it, but yeah. I think nowadays, the US girls win not because the judging is screwy but because at that point in the season they're the only ones who are REALLY prepared to take it out. In the past, definitely there was fishy judging but now I think it's more the type of field that is presented. Outside the US, the American Cup isn't that big of a deal on anyone's radar. Smaller countries like to go as a way to get great "big competition" exposure and experience, and it has a nice payout if you medal.