Marijuana testing is an arbitrary bar, but a fairly well-known one, so you do actually improve the distribution of your candidate pool by using it for low-skilled jobs with a large, undifferentiated labor pool.
Specifically, if it's known that these jobs test for Marijuana use, you're cutting off the portion of the applicant pool that knows this but still uses anyway, which stochastically reduces the likelihood that a candidate will eg show up to work intoxicated (fwiw, I'm a big fan of marijiana: I use it regularly, am a commensurately lighter drinker relative to my peers, and I'm pretty sure I'm making the right choice in terms of health and responsibility)
Like I said, it's an arbitrary bar and a path-dependent one, and a counterfactual equilibrium in which alcohol or another intoxicant was tested for would work just as well. But it's not irrational for employers to use it in certain labor market conditions. There are analogues in other labor markets, like requiring a college degree to be a firefighter; the false positive rate only really starts to matter if you start finding it difficult to find qualified applicants.
Tech jobs, by contrast, already have plenty of bars that their employees need to clear. In this context, any candidates swept up in blunt drug tests are likely to be false positives, and the use of the bar is a net negative for the employer.
Specifically, if it's known that these jobs test for Marijuana use, you're cutting off the portion of the applicant pool that knows this but still uses anyway, which stochastically reduces the likelihood that a candidate will eg show up to work intoxicated (fwiw, I'm a big fan of marijiana: I use it regularly, am a commensurately lighter drinker relative to my peers, and I'm pretty sure I'm making the right choice in terms of health and responsibility)
Like I said, it's an arbitrary bar and a path-dependent one, and a counterfactual equilibrium in which alcohol or another intoxicant was tested for would work just as well. But it's not irrational for employers to use it in certain labor market conditions. There are analogues in other labor markets, like requiring a college degree to be a firefighter; the false positive rate only really starts to matter if you start finding it difficult to find qualified applicants.
Tech jobs, by contrast, already have plenty of bars that their employees need to clear. In this context, any candidates swept up in blunt drug tests are likely to be false positives, and the use of the bar is a net negative for the employer.