Well, that has nothing to do with calling him a traitor, though. Bellhorn or Embree aren't traitors, IMO. Sell outs? Hell yeah. But not traitors.
Here's why I think some Yankee fans think Wells is a traitor: Andy Pettitte and Bernie Williams both received contract offers from the Red Sox, and both turned them down, feeling that it would be disloyal to their Yankee teammates to play for them. And so these fans think Wells should never have signed with the enemy.