Cannabis testing for hiring engineers- can corporations supersede the law?

Drug testing for marijuana has always been a ridiculous practice, as marijuana stays in the system long after its direct mental effects have worn off. It's also, very obviously, much more conducive to software development than alcohol, for example, which no one tests for. Furthermore, with increasing evidence that marijuana is not only good at treating certain conditions, but also good at preventing and inhibiting certain cancers and glaucoma, it seems that engineers who make a conscious, healthy choice to use marijuana are being told by potential employers that they need to sacrifice their health for work.

Yet some companies, particularly those that engage in work with federal agencies (especially the DOT), continue to test for marijuana as a condition of employement, even in states where marijuana use is legal or decriminalized. This seems to me to be an egregious overreach of corporate powers, and potentially even grounds for employment discrimination lawsuits.

What experiences have you had with employers that drug test engineers? If you work for a company that tests, how do you address the issue when hiring engineers, especially in legal states?