Well, so much for Danny's, "no one's untradeable" philosophy.
I don't think that's true, instead KG recognized that Danny would trade him if he thought it'd be best for the C's and made sure his agent took steps so that it couldn't happen without his consent.
KG signed for a hometown discount, I don't think he'd be happy being shipped off to another team because that contract makes him a better trade asset.
I think you nailed it. KG would have been a heck of an asset with that contract.
And I think that is why Pierce didn't get the trade exception. He was basically paid market value, or even slightly above (although it still was a bargain for the C's, because Pierce opted out of his old contract which would have cost them a lot more in luxury tax).