This is an interesting area. THC is a substance that can remain in the body with no intoxicating effects for several days. ingesting it on Friday night and it will remain until past the workday Monday. Traces of it remain in the hair until the hair is cut off -- months.
I have never heard of anyone claiming that those remaining traces cause a problem with cognitive or physical function for more than a few hours but in fact those traces remain. It is a logistical nightmare for the user as a urine test on Tuesday could prove positive even though THC has not been introduced for days.
But the cool thing about employment is that it is contractual. Sign a contract that says "thou shalt never use marijuana" and there you are -- it means what it says. Really the existing precedential case law only goes to the interpretation of the contract and it is legal to have a zero tolerance clause in a contract.
As a side note to consider. I worked with a very talented hopeless drunk. He did not use things at work that could be dangerous. He was paid for his brain. The SOB would "dry out" and come to work all shaky, sweating and nervous -- he was useless. He would have a couple at lunch and be extremely productive in the afternoon. I know some creative types that soar when stoned, and flail when sober.
Dependency is always sad. But they do not call it "self medication" for no reason.
No the case law will not change or alter. But perhaps our judgment of others will adapt.