I know the power of US, but in anyway,
From what I know, although I have to consult some more persons,
An online casino based in Antigua with assets in Antigua and servers in Antigua (or whatever country) does not have to issue a statement saying that it will not accept bets from US citizens. They only must stick to Antigua rules where the service is offered. They are free to accept bets from US citizens even if online gambling is ilegal in the US. That’s because they are not operating in the US.
If PartyGaming and others operate on other countries and do not have operations in US they don’t need to care with any US law, because it is not applicable to them. If they say they will not accept bets from US, it is their choice and not because they have to.