OK, you seem to be jumping around a fair bit on the roadside stop process so let's give you the scenario and you describe what you think should happen. Officer observes vehicle crossing the lane demarcation line giving probable cause to initiate a stop for operating a vehicle while impaired. Officer approaches the vehicle and observes pot-related paraphernalia or (even vaguer but equally valid legally) smells marijuana so it is possible the operator was smoking in the vehicle (analogous to alcohol rules). Driver refuses to perform field-sobriety tests for THC intoxication. Officer, having PC for impairment and passive non-compliance by driver, then....<please fill in the blank assuming no accurate roadside testing options and guidelines for intoxication limits>.

The same issue as above applies for corporate employment. Insurance companies will use State and Federal intoxication and abuse guidelines for setting the employee acceptable limits. They may choose a tighter standard if they wish, but ultimately the issue of impairment while working is something that the companies would be looking to the Government standards to define. The current on-site testing is not refined enough to show whether someone has used it at home (i.e. not impaired, a liability risk, or a termination offense) versus the operator of crane/tow-motor/sheet-metal bender/diesel railroad engine who just had an accident at work and might be a legal liability if they cannot test him to determine whether he was intoxicated with a legal substance at the time. That's just the most obvious case of risk.

Corporations with any risk of accidental injury/death, breach of fiduciary responsibility, classified or NDA work requirements, working with children/elderly, medical/pharmacy work, and probably some other insurance risks that I am not thinking of at the moment would default to all having blanket prohibitions against employees engaging in a legal activity during the hours spent outside the company? I.e. any marijuana use anytime which is the current testable standard is grounds for dismissal even though it is a legal substance? First accident/mistake at work for anyone who doesn't consistently piss clean equals automatic termination? Because, otherwise, it appears that they will have the same insurance/bonding/licensing risk issues if they cannot define impairment at the time of any issue.