The real reason is why should they bother testing for it? The commissions are concerned with fairness and safety in sport. It's baffling to me that they would test for ANYTHING if it wasn't directly related to those two core conceps, they are not there to enforce federal law.
i think its directly related to fighter safety. if a stoned fighter has a slower reaction time but can take more punishment due to the numbing effects, he may take an unnecessary beating. again, im pro pot for non athletes. but to dismiss weed as if it had no effects on you is kinda crazy. it would either enhance your peformance, or hurt your peformance. either one is bad.