Not surprising, just stupid. Who cares if the guy plays on a divisional rival? Acknowledging his accomplishments =/= rooting for him. If we're going to talk objectively about baseball, there's no reason to not be fair in our assessments of all players.
But then again, this is coming from someone who doesn't hate, or even dislike any sports teams, including the Red Sox. I root against them because if they don't do well, it benefits the Yankees, but other than that I have no problem with them. In fact, I really like a bunch of their players.