The biggest ignorance is what descheduling means.
A schedule I drug is defined as one that has no medical benefit. A doctor cannot prescribe a drug that is Schedule I without endangering his license to practice. In states that it is decriminalized for medical use, a doctor is simply saying that you meet the conditions to allow one to receive a medical marijuana card. They are not prescribing anything. Once it is officially moved from a Schedule I to a schedule III, a doctor can prescribe and thus would fall under the same requirements as most prescription drugs. Thus you would be protected from any employer retaliation from a positive drug test - as long as you aren't under the influence AT work.
For example, if you have a valid prescription for hydrocodone and you are not using or under the influence at work, you can't be fired for having it in your system. The thing with THC, it is fat soluable and remains detectible in your system for a lot longer than most other drugs. So in practice, what should happen, is that if you test positive on a random, no consequences. If an employer has reasonable suspicion you are under the influence and you test positve - there will be consequences.
The litmus test for reasonable suspicion is that the evidence has to be specific, contemporaneous and articulable. What this means is that you have to have definable evidence to test. Just because someone made a boneheaded mistake, that doesn't qualify. If someone makes a boneheaded mistake and they smell of alcohol, MJ, have bloodshot eyes, slurred speech, etc., then that would warrent a reasonable suspicion test. If you test positive, regardless of having a valid prescription, you can have consequences.
For example, I had a case were a guy tested positive for opiates. He had a valid prescription. The medical review officer contacted the prescribing doctor. They were able to correlate that IF the employee had been using the opiates as prescribed, his level wouldn't of been that high. Perfectly acceptable to part ways at that point.
Furthermore, the "At Wil employmentl" arguement is null and void if you have a valid prescription. ADA easily trumps this arguement. You can't tell me I can't take my prescribed blood pressure medicine, just like you can't tell me I can't take prescribed THC (whatever format taken).
On a side note, for a drug to be a Schedule I drug, there has to be no medical benefit from it. I'm not sure what fits that definition better than alcohol. The banning of this... well, we all know the outcome.
From personal experience, being in saftey for multiple decades and running our 10,000 employees' drug and alcohol program, I obviously would prefer no employees indulge. However, I would much rather have an employee who took a hit then day before to help sleep than them use alcohol to sleep. Also, I'm not niave. I would much rather have employee recreationally use THC the night before coming to work than alcohol.