Why would you offer to extend him as a way to make it easier to fire him if he's already under contract for 2013? That would indicate that Pioli will be back but on a very short leash. But if you have that little confidence in him, why extend him at all? Why not wait on an extension until sometime next season, to see how things are going?
Is it only because it's standard practice to not let a GM (or coach, IIRC) enter the final year of a contract? That protocol forces Clark to give him an extension, even though he's this close to not letting Pioli continue past 2013 anyway?