Why didn't Hoffman...? (Spoiler)

Spoiler ahead...



Why didn't Hoffman just get Thornwall off when he had the chance? When he had the tape of Callahan and Beers on the telephone, Hoffman could have just turned it over to prove that the two had framed Thornwall. That would have got Thornwall off, and ended Callahan's chances at being governor. Instead he let Thornwall suffer through further incarceration and another trial. It was only luck that the guilty gardener was caught trying to flee and confessed. It seems kind of harsh on poor Thornwall to be used that way by Hoffman just to prove his moral point.