Employers don't want people working under the influence of anything regardless of its legality. This will not have an impact on workplace drug-testing.
I don't think that will be true. Employers don't want people working under the influence of anything that can cause impairment, even if it's legal, and they have a right to prohibit employees from coming to work under the influence, consuming drugs/alcohol at the workplace, or even just bringing drugs/alcohol into the workplace. However, there's a pretty significant difference between testing abilities for alcohol and other drugs, ie, it is easy to prove or disprove that an employee has come to work intoxicated on alcohol or has consumed alcohol while at work. The drug test for marijuana would come back positive even if you've never violated your employment code by consuming at work or coming to work under the influence.
Of course employees would have no protection under federal law, but I bet people would bring unlawful termination suits against companies if and unless the company could prove there was use that violated the employment code.