Because there is an understanding, rightly or wrongly, that referees should know laws off by heart and therefore incorrect in law decisions should simply never be happening, certainly not at these kinds of levels. I agree with
@socal lurker that if he'd made a mistake in one team reducing numbers I could kind of understand, but to say that the injured player's kick should be forfeited is clearly wrong.
It has been said time and time again, but as a referee you have to be prepared for every possible eventuality no matter how bizarre, and even more so once you let to L4 and above.