From BR Bullpen
The disqualified list is used by teams to place a player who is not available to play for disciplinary reasons. A player on the disqualified list is not paid, although he remains property of the team. The list is used sparingly and only for very serious breaches in discipline. Recent cases include Francisco Rodriguez, placed by the New York Mets in August, 2010 after breaking a finger as the result of a fight with his father-in-law for which he faced criminal charges of assault, and Carlos Zambrano, disqualified by the Chicago Cubs after walking out on his team during a start in August of 2011.
Given the player is not paid while on the list, it is considered a form of punishment and its use is monitored closely by the Players Association, which stands ready to file a grievance in any case where it is suspected to have been abused.