I think FullTilt did not keep customer accounts separate from their operating accounts, which is a big no no.
PokerStars didn't have that problem so it was allowed to reopen to non-US customers
Um wtf, Pokerstars is NOT
open to US players.
And the issue has absolutely nothing to do with accounts. It's a complicated situation but in a nutshell, the US government decided to go after internet gaming by passing the the UIGEA back in 2006. The DOJ subsequently used this as the basis for prosecuting(persecuting) the big three poker sites last year, aka "Black Friday".
After "Black Friday", most of the major sites pulled out of the US market. However, there are still a few sites accessible to US players. Pokerstars is not one of them.
Fyi, Pokerstars is the absolute most trustworthy, dependable, and reliable poker site there is. Their customer service and integrity is beyond reproach. There's an old joke on the poker forums, "Who do you trust most with your money?", the answer: "1)Pokerstars, 2)My bank, 3)My wife".