Why Astros did not re-sign Pettite

Jayson Stark, over at ESPN.com wrote on Friday (12/8) why the Astros would not go the extra mile ($2M) to re-sign Pettitte.

The buzz around baseball is that MLB officials -- particularly commissioner Bud Selig -- weren't happy with the Astros for giving $100 million over six years to free agent Carlos Lee. So owner Drayton McLane might be trying to placate MLB's hard-liners by attempting to hold down the rest of his payroll.

My fundamental questions are why would Major League Baseball be purposely and/or covertly suppressing players salaries and does this border on collusion? Or is baseball exempt from collusion because their legislative monopoly? Either way, if this is true, it would seem that the Players Union might have a decent grievance, since the league could be influencing where players sign, and this does not sign like a true free agent market. Do you know?