It’s possible that USEF want to go after anyone who has used it, even if the horse is or isn’t a positive that day. Plus if it is short acting, maybe it needs to be dosed very close to the class (like most everyone here, I have no idea). If that’s the case, it’s pretty easy to avoid (not engage stupid autocorrect) testers unless you are the first class of the day. Word gets around, it’s not like everyone doesn’t know they are there. So no illegal drugs that day and the bad actors go back to business the next show.
If this is the case, I can see why regular testing methods won’t get the results needed to get at this problem…