Djourou signing that extension just gives him more security, and a better salary benchmark. No reason not to sign it really, if he knows we'll let him go anyway. I vaguely recall hearing the same as Tim, that some kind of option vested on his old contract. No idea if that's true, obviously.
Anyway, I understand why he'd want to go, I understand how frustrated those who want to see the back of him are with his last 15 months' form, and I can see no reason to stand in his way if he wants to go.
I just think he could still have a chancr of being a very good defender somewhere, even if it's not here.