A private bill is not unconstitutional and, in fact, they are passed all the time -- and offered by members of Congress of both parties and all philosophies. I heard another person say that Congress cannot pass a bill of attainder, which is true, but a bill of attainder has nothing to do with this case, either. Such bills are for punishment, not to protect life.
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.