Sure, I will explain it to you.
Metrolinx has this inconvenient thing called "liability".
It over rides your right to get high.
Imagine a scenario where a westbound GO Train driver blows through a red light and slams a commuter train into an eastbound freight train. 50 people are killed and hundreds injured. They arrest the driver on the scene and take him for drug and alcohol testing. He is found to have cannabis in his system. He swears that the last time he got high was 3 weeks ago.
Now the lawyers get in on the action. They argue that the driver had cannabis in his system and it impaired his availability and caused the driver to run through the red signal and slam into the freight train.
They launch millions of dollars in lawsuits against Metrolinx and the whole thing drags out in court for 7 years. During the trial, the lawyers for the victims bring in a pile of paid "experts" who go on and on about how even minute traces of cannabis can impair reaction times, blah blah blah.
In the end, the jury, being composed of such knowledgeable individuals [ sarcasm] finds Metrolinx to be liable for hundreds of millions plus tens of millions in legal fees. The case gets appealed to the Supreme Court, which costs another 50 million in lawyer fees and drags out for another 5 years. And on and on and on.
All so a train driver has the right to get high.
In such scenarios, juries are composed of morons who will award hundreds of millions of dollars because they figure that someone has to pay.
So Metrolinx figures that the public has a right to ride on a train or bus being operated by a guy who does not have recreational drugs in his system. They enact a policy which states that if you want to work for them in a safety critical position you cannot be a drug user. It's their right as an employer to make it a qualification for the job.