Thats strange, though if they hold it would probably be for some investigative purposes and not really for cashing except the government and the central bank know about it. But this is seeming ridiculous, maybe they use it to sponsor their spies on the darkweb
Yeah I also think same like you, FBI use bitcoin to make payment to their spies, but I do not believe that FBI hold the biggest number. If it is true then in short bitcoin price is in the hand of FBI, and think what, it means bitcoin price is in the hand of America. Well it is strange for me like you said. But I believe FBI have sufficient amount of bitcoin that they will use for investigation purpose.