As others have pointed out, Ulbricht has not claimed that they are his bitcoins. In fact he would be undermining his own legal defense if he claimed the bitcoin

As others have pointed out, Ulbricht has not claimed that they are his bitcoins. In fact he would be undermining his own legal defense if he claimed the bitcoins were his, so the government is taking them unopposed. So if he were found innocent, probably part of the basis for finding him innocent would be that he successfully convinced the government that those were NOT his bitcoins.