It's not just so that employers can find out who's using drugs or drinking on the job; it's to make sure that workers are safe at work. With the absence of any federal or provincial definition of a safety sensitive position, it's left to workplace parties to work it out and, ultimately, to litigate this. There are literally hundreds of cases every year fighting about whether a particular position is safety sensitive or not. The statutory definition wouldn't answer everything, but it would give guidance, and that's what needed.
On September 14th, 2017. See this statement in context.