WASHINGTON (MarketWatch) — FXCM has been banned from operating in the U.S. after the Commodity Futures Trading Commission found the retail currency broker had an undisclosed interest in the market maker that consistently “won” the largest share of FXCM’s trading volume – and thus was taking positions opposite its retail customers. The CFTC also found that FXCM willfully made false statements to the National Futures Association in order to conceal FXCM’s role in the creation of its principal…