The U.S. can tax any income above a certain level. Gambling winnings are generally considered income, on or off-shore. However, it would be difficult for the U.S. to track such winnings, as they wouldn't be reported to the IRS as such winnings are inside the U.S.
Thus Congress, ever the conservative institution, determined that since it couldn't guarantee under WTO rules it would be able to continue wetting its beak, better to pretend to social conservatism and ban the whole thing, sending the gamblers to unregulated bookies and regulated casinos.
thank you for adding to the discussion of the question I posed.
Very informative thread going here, for those of us who don't understand the ins and outs of the law.