The moment there isn't federal law to lean on, I hope for and expect court cases predicated on the fact that there's no basis for an employer to care more about whether someone has smoked cannabis in the past thirty days than they do about whether that same employee gets blackout drunk every Friday and Saturday night - nor for that matter if the person responsibly drinks a couple beers after work some nights. (Or is someone pushing to detect alcohol use within the past 30 days as a reason to disqualify employment?)
Neither of those details of their lives speaks to someone's sobriety at work, and the basis for considering marijuana usage as somehow "worse" is rooted directly in the racist basis for policies enacted at the very start of cannabis prohibition.
The reality is that drug tests just like felony checks are very good filters for bad employees.
If this is true, drug testing should start at the CEO.
Edit2: Hanging onto this for 2 months before replying, or just like trolling through old cannabis discussions looking for an argument, or...?