Online
gambling as a whole is not allowed in the USA. Its all about the legal interpretation of something called the wire act from the early 1960`s, which was about using telephones in sports betting. This has been enterpreted by judges to also cover using the internet for playing casino games or playing poker.
So why then do some sites still accept US players? Well because they are situated outside US jurisdiction, so they basically just dont care about US legislation. As I understand it, PokerStars,
partypoker and
888 had assets in the US and therefore kind of had to flee the country to not have these confiscated.
Its kind of a modern prohibition period really and can easily be compared to the 1930`s ban on alcohol. And as with that ban I am sure, part of the background for this is the strong religious communities, which exist in the US. As we all know, alcohol and gambling is both consider immoral by many religious people.