The vast majority of softwares do not allow ANY players from the USA.
e.g. This one is Playtech - all their casinos are unavailable in the US.
The main softwares who do still take Americans are RTG (RealTime Gaming), Top Game and Rival. But do your homework on each casino as some of them are rogues.
There is also BetOnSoft (relatively new) which all seem to be OK so far.