Isn't it the case that he could not spend the Bitcoins? The original Silk Road account obtained the funds illegally. He obtained them from the Silk Road account illegally. As long as the funds remained as Bitcoin the Feds didn't know who he was or where they existed. Once he tried to exchange the Bitcoin for something usable like dollars his identity can be obtained.
Laundering Bitcoin can likely be done - it just isn't as easy as everyone assumes.