Because the runner was off on the pitch, he gets credit for the steal.
If Varitek had done something like that on an attempted pick-off at first base [w/ the runner not initially running], lost the ball, and then the runner advanced, the catcher would likely be called for an error.
As good as the Red Sox were in defending the steal [8 in total that game, including one by the opposing catcher], they should have kept their infielders in position, not made any throws, and if they were lucky enough, the official scorer might have scored them as defensive indifference.
Ted