In this case there's abundant evidence of a criminal enterprise involving the sale of drugs, attempts to purchase the services of hitmen etc. etc. So 'Dread Pirate Roberts' only likely defense is to claim something along the lines of 3v1l H4x0rs taking over his computer for the last several years while he played minesweeper or suchlike, and disavow any awareness of of or custody over the money; otherwise he'd have to explain just how he came to be in possession of several $m of BTC that was sent to Silk Road.

This is interesting as an example of BTC's strength being a vulnerability. Suppose you were arrested in a warehouse with a pile of cash and numerous bricks of cocaine, AK-47s, etc. Of course the government would claim you're a drug dealer, but you could attempt to convince the court that you had won the money in Vagas/sold a great deal of pizza/whatever and been kidnapped by evil drug dealers on your way to the bank, only to wake up alone and confused next to your cash in the warehouse just before the cops arrived. An unlikely story to be sure, but there are many legal means through which you might come to possess a large stack of currency (and which might also provide you with a much-needed alibi, eg video of you winning big and leaving the casino only to be waylaid on the strip by masked bandits). Where BTC is concerned, though, the blockchain itself is proof that the money was sent to the receiving address given out by Silk Road in exchange for contraband, so the government has a prima facie case that it consists of revenue from a criminal enterprise.