Because there is so much money involved, no one will leave on their own and not collect the money. They can negotiate a different price if both parties agree that they should part ways. If he was going to just walk away, The new team , St.John's in this case, would have to offer Hewitt incentive enough to walk away from a buyout.
I'm just guessing here, for the most part. Not sure if this makes sense or not. I'm no expert on contract law, Tha Kid?