It's an ESPN insider-only article. Basically, Peter Gammons reports that the Pirates had a deal in place with another team (perhaps the Giants?) offering a better package of prospects for Adam LaRoche, but that they took the Red Sox' package because the Sox assumed his whole salary. I wouldn't presume Gammons' reporting is infallible here, but I look forward to hearing more about whether this is true. It would be infuriating if it is, obviously, even in a relatively minor trade like this one.
It might not be wise to assume the worst here--it could just be that the Pirates were offered a prospect package similar in quality to the one the Sox offered, and the salary thing was a tie-breaker. I hope someone can get to the bottom of this, though.
UPDATE: The Pirates reply to the accusation that it's a salary dump. There apparently was another offer, but it wasn't from San Francisco.