According to WSJ:

Federal authorities say online gambling is illegal, but Internet-gambling advocates say it is a gray legal area. In 2006, Congress passed a law making it illegal for banks to processing (sic) payments for unlawful Internet gambling. Critics say the law is unclear.

So, basically, the government insists it is illegal. They just don't usually bother to interfere. Although under Obama, the feds have started seizing accounts.