Apparently employers in the US drug test their employees

I found this out a few months ago and I thought this was absurd. It's not your employer's business what you do in your free time. Why in the world would an employer expect to be privy to such knowledge? Is it a by-product of their silly war on drugs from the 80's?

I've worked dozens of jobs and never been drug tested in my life. What are your thoughts on this, and could you see it coming to Canada?