If I’m an employer why shouldn’t I know if employees/potential employees are using?
And in return, your workers should demand than you drop your pants and piss in a cup in front of them, so they can be assured that their employer is drug free. It’s a two-way street.
And if you’re an employer who thinks it’s a crappy idea, why shouldn’t you let the gubmint force you to do it?
If you production is as good related industries and your workplace is as safe as related workplaces, why should you care?
Besides, you only catch people the have used cannabis. The other drugs leave the body post haste. Evidence of cannabis stays in the body and is detectable for a month after the use. That means that 718 hours after it has caused any effect whatsoever.
So, what would be the point? 
It would be very apparent if an employee had a problem drug or alcohol habit, and they could be easily weeded out by the basic common sense workplace policies. As an employer, I would NEVER drug screen my employees. If someone shows up for work, does a great job, and is always sober on the job; I couldn't care less what they do on their own time away from work.
If they aren't using on the job, it is not an employer's business what one of their employees does in his spare time.
I've long since even having more than a beer every couple of months, if even that much, but it galls me more today than it did 15 years ago that someone thinks they have a right to make me piss into a cup for them. Next time my company demands it of me, I'm really likely to offer my sample (sans bottle) to the highest executive I can find.