Drug testing would be for audits or other sensitive shops/assignments where you would also likely get a background check. If this client or the nature of the assignment is that sensitive to the corrupting influence of after-hours behavior and if that is what they would require of employees, it is all part of the screening process.
Think about it. For the basic work most of us start out doing, all we do is fill out the application. If you are working as an outside contractor around a business with high theft items or involved with regulated items or auditing protocols in testing or handling across any number of industries or settings, the contracting companies need to field a contracting pool that reflects higher standards than the general population for sensitive work.
Minor traffic violations are one thing. Check fraud, a shoplifting incident, insurance fraud, you get a record like that, nobody wants that in their business. (I have been contacted by a couple of companies to pursue different assignments or opportunities in a separate division that pays more for the type of work. I will probably look into this once the kids are in school again).
But then again, the electorate has lower standards it seems.
Take Florida. I think in his former life Rick Scott was involved in one of the largest Medicare fraud cases in U.S. history. He plead the fifth like 60 or 70 times in congressional testimony. I think his board of directors gave him the boot, not sure. I wonder how much they paid him to leave? Time to turn politician.