I would have to believe one of these applied:
Rule 3.01 (formerly 8.02): Prohibits intentionally discoloring or damaging the ball with any foreign substance (dirt, paraffin, licorice, sandpaper, etc.).
Rule 6.02(c): Prohibits pitchers from applying foreign substances, having them on their person, or attaching items to their hand/wrist (e.g., tape, bandages).