At the start of the video the little kid and the instructor were quite close and if I was the instructor I maybe would have thought the boarder's path too close for comfort. After all, little kids have a habit of random turns and as stated earlier decisions are made in split seconds. So I could understand him warning the boarder off.
That said, pulling his pass was a complete over reaction. To me the boarder seemed quite respectful - if he keeps that up he's going to be ruining snowboarders' reputations ;<)