> I'm still flabbergasted that he was using servers in the U.S.
He may have used some servers in the U.S. but the server the FBI grabbed was overseas. From the complaint, page 14, item 22:
In particular, the FBI has located in a certain foreign country the server used to host Silk Road's website (the "Silk Road Web Server"). Pursuant to a Mutual Legal Assistance Treaty request, an image of the Silk Road Web server was made on or about July 23, 2013, and produced thereafter to the FBI.
There's a list of U.S. Mutual Legal Assistance Treaties here. Who's got a guess?