I have been asked this question 3 times in 10 days. Last year, I was curious about it so I looked it up. Thought I would post it here in case you were curious.
In 2006, Congress tried to do away with internet gambling. They passed the Unlawful Internet Gambling and Enforcement Act of 2006. Bush signed it, but it isn't referenced in the Bush Library anywhere.
Somebody was worried the statute would make all fantasy football leagues for money illegal so they made an exception to the statute. I don't think Congress intended to create something like we see today, but fanduel and draftkings seems to follow all the requirements.
31 U.S.C. §5362(1)(E)(ix) provides that the term Bet or Wager does not include:
(ix) participation in any fantasy or simulation sports game or educational game or contest in which (if the game or contest involves a team or teams) no fantasy or simulation sports team is based on the current membership of an actual team that is a member of an amateur or professional sports organization (as those terms are defined in section 3701 of title 28) and that meets the following conditions:
(I) All prizes and awards offered to winning participants are established and made known to the participants in advance of the game or contest and their value is not determined by the number of participants or the amount of any fees paid by those participants.
(II) All winning outcomes reflect the relative knowledge and skill of the participants and are determined predominantly by accumulated statistical results of the performance of individuals (athletes in the case of sports events) in multiple real-world sporting or other events.
(III) No winning outcome is based—
(aa) on the score, point-spread, or any performance or performances of any single real-world team or any combination of such teams; or
(bb) solely on any single performance of an individual athlete in any single real-world sporting or other event.