They also don't give a shit about you or the truth. They just wanna cover their asses. Unionized? Federally covered, or provincial labor code? Soon will be a human rights beef, but not till they change the basic law, unless a prescription is involved, then hope. Maybe someone is organizing and can help with a court challenge, case will be heard after legalization, but might give the greedy bastards pause.
Your heath Canada paperwork is your greatest weapon here, use it. Labor Canada has ruled on this before, go to the website and search for precedent, I think I saw something you would like there. Health issues on this one should be covered federally, since the program is federal, go have a look there too
Tell'm yer going to the CBC and public big time if they try to ax ya, make it very painful indeed, contact me for helpful suggestions in this regard. Maybe a boycott of such employers by customers might get their attention, perhaps I could send them an email. "It has come to the attention of the national organization...." A website with a target list might be useful too, this is a big issue with lots of folks fucked over.
Hope it helps. Legalization will help the most with this bullshit.
Good luck and let me know how it turns out
Drug testing for cannabis is complete bullshit.