I think employment-based drug testing has lowered recreational drug use far more effectively than the law ever has. Even if drugs were legalized, I doubt many companies would alter their drug testing policies. Heck, more and more companies are testing for nicotine and tobacco is perfectly legal.
And medical marijuana? Good luck with that. Some California defense contractors learned this lesson the hard way when they bypassed their corporate health care providers to get medical marijuana prescriptions from a private physician. They were fired when they popped on a drug test.
You are right. Company drug policies will screen out users. Transportation workers (drivers, etc.) are under federal drug testing requirements.