The problem is that almost all available tests for the presence of cannabinoids return positive results long after the effect of using marijuana has passed. In America in 2017, as far as rules-makers are concerned, pot should be treated like alcohol. The goal should be to ensure riders aren't under the influence.
A test that bans a rider for using pot days before racing doesn't improve safety, it's just out of date moralizing.
I apologize for the error-riddled (but stimulating) opinion piece I wrote and posted earlier today, inspired by Dalton Gauthier's ban, which came after he tested positive for marijuana use after the Charlotte half-mile.
PS... For what it's worth, when I make a big mistake like that, I dock my entire salary for the day.