L4: your response regarding insurance companies "driving" these tests (pun intended!) certainly has merit...but let's not forget about the long-running love affair both entities have had for years whose relationship negatively affects many people in their daily lives!
All I can add is this...after retiring from "full-time" work, in 2008, I relaxed for a year, and literally did nothing more strenuous than cut my lawn. However, I was still rather young, and also felt guilty sitting home, while my wife still worked her part-time gig. I was in one of the local auto parts stores, picking up some supplies for a project, and the store manager, who I kina' knew, asked if I would consider working part-time for the store, driving a delivery truck.
I felt that a little "pocket money" wouldn't hurt, so I took the job. And in 2009, at the age of 57, I had to submit to a drug test, for the first time in my life. It was either that, or no job, and the company's regional manager told me that it was for insurance and liabilities concerns. The store didn't want any possible lawsuits stemming from an intoxicated individual being involved in a crash, while on the job.