Haven't looked at it in a while, but pretty sure that if (i) you are physically located in the United States and (ii) transmit any information (or the site you're using relays it) across any U.S. interstate lines of communication (would include telephone lines or DSL/cable/broadband pipe), which is virtually a given, to bet on sports, the reasonable argument is that you're violating the existing patchwork of federal laws prohibiting sports gambling.
That's not to say it's a worthwhile use of the feds' time to go after the people placing the bets or there is any political will to do so, but they could.