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.
If they can't access them, how did they move them to this address that they publicly announced?