Same as testing positive for alcohol. When they say “drugs don’t work here” they mean it.
I worked for years at a company that tested for drugs. Pre hire/employee, didn’t matter. You pop positive for Jack Danial’s or Bud Light and you were fired on the spot, or not hired if it was a pre-employment screen.
There are lots and lots of jobs that require drug screening before employment, and random testing once employed. Just because a drug is legal, that doesn’t mean you can use it and keep your job.
The huge difference is if you wait a few hours, that Jack Daniels or Bud Light won't show up in a screening. Same with Cocaine, Heroin, Meth, etc. With pot, you're looking at up to 30-90 days for it to clear. It's not a fair process, and if a state is legal and the employee or candidate is doing it on their own time, an oversensitive test shouldn't disqualify them.