A drilling company, an oil company, and a trucking company might all have legitimate reasons for drug testing, given that they operate within dangerous environments using heavy machinery. However, Walmart is a cheap store where the most difficult task is running a package over a scanner or pushing a broom. There is no legitimate reason for them to require drug testing as a regular thing. Now, if they want to drug test a current employee as a condition of continued employment if they suspect that the employee IS HIGH AT WORK, then it might be reasonable, but otherwise, it is none of their business if an employee likes to spark up off duty, especially in a state where it is legal. If Walmart is randomly drug testing employees without cause, then they are idiots.
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.