Because they're fickle idiots, Chirpy?
As I said before (and King stuck it in his sig), there's little or no difference between what Adebayor, Nasri, Fabregas and Henry all did, yet two are heroes and two are villains.
The honest pair, who admitted that they wanted to move on to clubs which they felt were more ambitious and for a better deal, are hated.
The two liars, who claim to love the club, yet played **** for a year and moped about, feigning injury and looking disinterested, are beloved of the faithful.
Adebayor admitted that it was flattering to be linked with Milan, but I don't think that he even put in a transfer request.
He scored 16 goals in his last season at Arsenal, but his level of effort was criticised and he seemed to fall out with the likes of Bendtner and Nasri, so he was sold.
Not sure that's the same as Berbatov's refusal to play and last minute dash to Manchester, but it's certainly something that he's been tarnished for.