Usually the employee handbook will say employees are subject to pre employment or random screens if you are. It sounds like they just have a reasonable suspicion rule where they can test you if they think you're f'd up on the job. Just stay sober at work and I think you should be OK.
The above post is fictional and should in no way be taken as truth.
If I were to grow cannabis, I would only do so to supply myself and my close friends. Doing this would take myself and my friends out of harms way by knowing the source of our cannabis, as well as take hundreds of thousands of dollars off the black market annually. It would also lower the demand of black market cannabis, and would ultimately help keep cannabis out of the hands of minors.