As long as Cannabis and its essential oils remain a Schedule I it is not considered a script for medical cannabis patients and instead a Doctors Recommendation.
This means that at any given point in time it is perfectly acceptable for one to be turned down for or fired from a position simply for having used cannabis for a medical condition. This includes the use of non-psychoactive topical cannabis.
Through lowering the Schedule of Cannabis from I to II or lower a PCP can then give a script to the patient that would allow them to have some form of ground to stand on when an employer begins questioning them about personal medication.
Take a quick look at the LD50 of Cannabinoids per effective dose VS that of any other pain medication used for people with spinal conditions and cancer.