As the Department of Justice prosecutes alleged manipulation of the foreign currency spot trading market, a trio of London-based foreign currency traders who worked for Citicorp, Barclays, and JP­Morgan Chase agreed earlier this month to come to New York in July for their arraignments.

But why face federal prosecution in the Southern District of New York, when U.K. investigators gave the traders the all-clear, and their lawyers have said the U.S. could lose an extradition fight?