They, along with several other sites, have decided the market was too big to drop out of.
Not sure that is an accurate explanation, DJ.
Those sites which have banned American players have, as far as I can tell, done so for one of the following two reasons:
1) They came under pressure from their bankers, because banks are primarily responsible under UIGEA and most large banks have offices in America and are vulnerable.
2) They are publicly quoted companies. Stock market rules (certainly in the London market, where many of these companies are registered) require, as part of business ethics, that companies shall not flout the law in any major jurisdiction where they trade.
I don`t know of any site which has banned Americans as a business decision. Difficult to see how that would make sense, even if Americans were only a minority of their customers.
The sites which still allow Americans are those which are privately owned, do not use American bankers and have no physical presence in America, so that they can feel reasonably safe from possible harrassment.