Actually, contrary to popular belief, online poker isn't illegal in the U.S.  I dare someone to show me one law on the books that makes online poker illegal.  There are still many sites that offer online poker to U.S. residents.  Most of the laws that FTP and PS violated involved bank fraud and money laundering.  FTP was accused of running a ponzi scheme because they couldn't pay back their players.  PS struck a deal with the DOJ to purchase FTP assets and part of the deal involved them leaving the U.S. market forever.  That's how it came to be that US citizens can't play on Poker Stars and Full Tilt Poker.

This was from a 2011 Forbes article.

"On the Friday before the Christmas weekend, the Department of Justice revealed that it had changed one of its most important and long-held positions on Internet gambling, stating that the federal Wire Act of 1961 only applies to sports betting. The new position on the Wire Act marks a huge shift for the Justice Department, which has long relied on the law when asserting that all forms of Internet gambling, especially online poker, is illegal.

“The Department’s Office of Legal Counsel (“OLC”) has analyzed the scope of the Wire Act, 18 U.S.c  § 1084, and concluded that it is limited only to sports betting,” U.S. Deputy Attorney General James Cole wrote in a letter on Friday."