Ainge says that he's not shopping Walker, and I believe him. He doesn't have to. Walker's agent Mark Bartelstein is working the phones hard looking for a place for Antoine to land. Which is his job. He knows that there isn't much interest from teams with cap room so the most he can get for his client elsewhere is the MLE. Thus the sign-and-trade option becomes his best way of driving up the price. If he were to resign here, Bartelstein wants the price to be as high as possible.
Still, Ainge is no fool. He knows that any deal higher than the MLE is going to have to come from the Celtics and its just a matter of weighing the pros and cons of sign-and-trade offers worked up by Bartelstein.
Walker's agent also represents Blount and don't forget that the trade kicker actually makes it more benefitial for Blount to get traded. Look for that to happen if only for that reason.
Update: For fun, lets make a game of this. See "Being Mark Bartelstein" on the message board.