As an example of what I am talking about:
An employee at WalMart is involved in an accident on the job.
It's clearly work related, no employee malfeasance caused it, the employee makes a workers comp claim.
WalMart insists on a drug test. The employee comes up positive for THC.
The employee was not observed to be under the influence, etc - but WalMart and the insurance company will claim that the employee is "on drugs" and deny the claim. If the guy had gotten blasted on scotch every night that week, no foul, but if he had a toke a couple of times before bed or whatever, its an issue.
Law enforcement does not currently have a good way to check drivers for being high for example as well. All of this is uncharted territory.