, you can see that the father caught the ball, and gave it to the boy in order for the boy to throw it back. We don't know how the sister ended up with the ball, it happened while the guy crossed home plate. So yes, it was the boy's ball to throw back, but if he didn't do it, it was important by tradition to get the ball thrown back before the next batter came up. Probably should have given the kid a little more time. Actually, the guy probably should have just thrown the ball back, but he wanted to share the experience with the kid.
But this is NOT about someone being mean to a boy by stealing HIS ball and throwing it back.