Well, see, the thing is, they don't. No one gives two shits about weed. Ran into a coworker today vaping skunk in the parking lot. Hell, 20-years ago an employment recruiter told me they no longer tested for weed because they'd lose 80% of their people.
In any case, if you can't lay off whatever shit you're hitting for a few days to pass, you got a problem. And yes, some employment requires the most straight and narrow sorts of people. Grocery stores are not those jobs.