There is a "fatigue clause," where if a player played a certain amount during the regular season the player cannot participate in winter ball. It seems that typically teams lift this clause and allow players to join the winter leagues, but this year they refused to lift the clause for Cabrera. The implication is that Cabrera is a trade candidate, perhaps in a Johan Santana deal.
More info on this extreme fatigue clause can be found here. Thanks to Carlos S. for the links and translation and to Keith Law for translation.