A bill of attainder has nothing to do with this case only because Congress didn't judge from the Capitol, instead simply playing with jurisdiction. It would have been a bill of attainder if Congress had said to resume feeding or give guardianship to her parents (who would resume feeding), because that would be depriving Michael Schiavo of his legal rights as her guardian.
You are wrong. Bill of attainder is not open to debate. It is a legislative act in which punish is imposed against a person -- denying the state the right to kill by starvation a person is the opposite. Whether that includes denying Michael Schiavo that right makes no difference, either.