In terms of the FBI cashing out the silk road BTC, they can only do that once it has gone to trial and a verdict has been decided.
There is also circumstantial evidence that while they do have access to DPR's personal Bitcoins, and they have a dump of the disk with Silk Road's escrow wallet, they do not have the key to access it. Thus there is a good chance that those BTC are gone forever rather than in the US government's hands.