Rule 2.00 "In establishing the validity of the catch, the fielder shall hold the ball long enough to prove that he has complete control of the ball and that his release of the ball is voluntary and intentional. If the fielder has made the catch and drops the ball while in the act of making a throw following the catch, the ball shall be adjudged to have been caught."
In this instance the key phrase is dropping the ball voluntary and intentional. Which was not the case in this catch. Feel bad for him, and the 3 runs, kinda adds insult to injury on the play.