There are and should be limits to what an employer demands of their employees in their off duty hours.
However, that being said, while a test can determine if someone has THC in their system, unlike alcohol, it cannot determine if it is enough to cause inebriation.
The best result might be a medical prescription that indicates how much of what kind of marijuana a patient can smoke when off duty, and still be functional at work. Most prescriptions are scheduled, not “as needed”, so this isn’t too unrealistic. Doctors would use a weight table, suggest the marijuana before, during or after a meal, as well as asking the patient about their own experience, how long a high lasts for them.
Im regards to all drug testing, whether pot is legal or not, states/feds should specify impairment levels.
Impairment testing is a common sense solution to the problem.
Impairment Tests: An Alternative to Drug-Testing in the Workplace
While I'll argue the point that no one, not even your employer has any right to intrude on your privacy enough to determine what you do on YOUR time, I'll also argue they have EVERY right to determine whether or not your fit to do the job they hired you for.
Otherwise, they should be paying you 24/7 to follow their WORKPLACE policy.
I don't believe this is an accurate comparison. Just because .10 is "legally drunk" in some states doesn't mean that a person who blows .06 into a testing device isn't drunk or impaired. I've seen people become completely snookered after two glasses of wine. No way they'd blow .10 into a tester, yet they were drunk.
I've also seen people who can drink an awful lot, blow .10 into a testing device and appear to be perfectly sober. (They're what you might call "functional drunks.")
So any test for THC to test the "level" of inebriation would by nature be terribly flawed the same way an alcohol test is.