I know they can. It was more a question of whether they do or not. I haven't worked for a traditional employer in almost a decade and haven't worked for an employer who did drug testing for probably 12 years at this point. With the social shift and marijuana losing its stigma, I wondered if it still actually mattered to most employers.
I get drug tested at least every year. If you use drugs in my business, you lose your job immediately since insurance companies will not tolerate drug use. BAM!
A recent story online was about a large number of interns who had been hired in a summer program by a major company. They objected to the company's dress code and all but one signed a petition to that effect and presented it to management. The company fired all but the one person who did not sign the petition.
I am old school myself---at least as a first approximation. If you present me with non-negotiable demands, I will most likely agree---and that is the last time we will speak about those demands, because I will reject you as well as your demands. (After all you have given me no choice---you said they are non-negotiable.)
Benedict Arnold 