Because retention of metabolites and active ingredients in marijuana lasts so long, how will you determine in a non-subjective (quantifiable, repeatable) way what levels constitute intoxication?
Keep in mind the alcohol analogy. I have seen drunks with a B.A.C. well in excess of .20 who seemed 'a little off' but were clinically determined to be fully loaded.
There are hardcore users who give the appearance of being conversant and functional, but upon closer examination and testing prove otherwise.
Will the claim be made that some can 'handle their weed' better than others?
I see fertile ground for lawyers to sprout, but the issue should be resolved.
I also think, every employer should be allowed to retain the right, agreed to upon upon hiring, to have an employee tested in the event that employee is involved in an accident (and the employee should have the option to send a sample to another lab if they so desire).
Otherwise, as an employer, why would you want the liability exposure?
One last thought: Just because something is legal, that does not necessarily mean it will be universally regarded as desirable, or for that matter, right.
I remember an interview with a large worldwide American company where they asked an HR executive why they only drug tested American employees. Her response was that only the Americans would put up with it. Land of the Free, indeed.