Why is U.S. customs law helping Amazon and China, and not U.S. manufacturers and retailers? MarketWatch 19 mins ago Charles Benoit © Getty Images/iStockphoto OUTSIDE THE BOX Last year, Amazon AMZN earned a staggering $21 billion . And much of that profit came directly from Amazon’s position as China’s online marketing machine. China now accounts for roughly 40% of all Amazon sales, and an estimated 75% of all new sellers on Amazon are Chinese companies. Unfortunately, domestic U.S. manufacturers and retailers are on the losing end of this arrangement. Beijing already massively subsidizes its state-owned factories—which allows them to charge…