See my reply to Seanlahman

MLB players with a major league contract and more than three year's service cannot be sent down. Period. They can only rehab. Says so right there in the Collective Bargaining Agreement. Those who get three year major league contracts don't get sent down either, because financially no one is going to play major league minimums to keep a player down on the farm, and it is very difficult to cut a player with those kind of dollars on them as well.

Players can be released, but that is not the same thing as being sent to the minor leagues, where they are still under contract to the MLB club.