The clue was in "Prisoner's Dilemma". Read the rules of that again. As others have said, the court is after Knox, not him. He will have been told (or more probably, hinted at) that if he blamed everything on her but admits the crime, then the courts will show leniency (couple of years inside maybe) and say that he was seduced by her into committing the crime. The Italians get everything: they get to blame Knox, but without the inconvenience of her actually being there to defend herself. If he continued to protest his innocence and is found guilty he will get thirty years. What would you do?