Because he wasn't sure of the answer, i.e. that a walk is still a live ball. After having gone through this with my son as a 9 year old and parents screaming "he can't do that" you have to explain to them the ball is still live, because the vast majority have absolutely no clue.

Just like idiot coaches even some at HS level who immediately after a walk, try to call time and start walking towards the mound without any regard for the situation.