Baseball and Steroids

The New York Yankees are apparently considering whether they will try to void the contract in effect between the team and First Baseman/Designated Hitter Jason Giambi, because of his apparent admission that he took steroids: http://sports.espn.go.com/mlb/news/story?id=2876608. Maybe I'm missing something, but didn't Giambi already admit this? Why is it an issue now?