Because regular anti-doping testing during childhood and adolescent sports seems like a bit of overkill.
Interesting problem. I admit I'm not sure what the right thing is when it comes to hormones and competitive sports.
Someone undergoing hormone treatment at that age is probably getting even stricter testing than most anti-doping testing just to make sure they are transitioning safely.
I wouldn't have thought that most athletic committees would allow athletes to use high levels of testosterone and compete in amateur athletics, even if the testosterone is being used for another reason like gender re-assignment, and not performance enhancement.
Athletics have skirted this issue in the past, even without gender issues. Many athletes are prescribed hormones and drugs for recovering from surgery or injuries but are not allowed to compete in the sport while under the influence of the drug. Certainly, the therapeutic drug is being used for a benign (or benevolent) purpose, but it's still a banned substance when it comes to competing.
I'd have think this would be similar. I think the rule that "you can only compete as the gender (or sex) you were born with" is not right. Obviously there has to be some control so that somebody isn't seriously injured. I think it becomes a lot dicier when people born as men but have undergone gender re-assignment want to compete against women than it does in the reverse like in this case.
When I was undergoing chemo (was 14) and had to be put on several steroids and testosterone after the treatment ended, I basically had to end my HS tennis career after my freshman year because I was breaking the anti-doping rules with my treatment. Which, to be fair, was probably the right thing to do - because I was able to build muscle much quicker and train way harder than they could, even after suffering pretty bad after-effects from the chemo itself. Testosterone supplements at that age can do some insane things, and I was competing with other guys, not women.
It's a dumb rule, the league needs to change it, and for any transitioning competitors, they should have their testosterone / hormone levels monitored and used as an appropriate measure for competition. This does not seem complicated.
(MMA & Boxing are the two sports where this becomes a much more thorny issue due to the fact that actual damage is being dealt to each other - but I have zero idea how the hell to figure that one out)