You nailed it. It's the perception of the Sox as an organization and the way they have usually bad mouthed players that were headed out of town. And that goes back a good solid decade. Fisk, Onedog, BlackJack, Robin, Ray Ray, etc.
I have noticed recently that KW seems to be trying to buck that trend. He's been trying to just let Magglio's performance in a Sox uniform speak for itself and not get into a war of words with Bore-Ass. Even with Koch, he had nothing but positives to say about the guy other than something along the lines of "it just didn't work out for him here."