David Meister, the director of the CFTC's Division of Enforcement, said it's illegal for Americans to buy and sell commodity options unless they are listed for trading and traded on a CFTC-registered exchange.

"The requirement for on-exchange trading is important for a number of reasons, including that it enables the CFTC to police market activity and protect market integrity," he said in the statement. "Today's action should make it clear that we will intervene in the 'prediction' markets, wherever they may be based, when their U.S. activities violate the Commodity Exchange Act or the CFTC's regulations."