Barrett was in the process of attempting to field the ball. Even if he was just setting up waiting for the throw, catchers aren't called for interference the way a regular infielder would be if they get in the way of a baserunner when they don't have the ball. The flip side of that is baserunners trying to score can run into the catcher and try to knock the ball out at the plate.
If you remember a few years ago, ARod tried to karate chop the ball away from Bronson Arroyo as he ran to 1st base, that is not allowed either
