I mean, that's the thing, it's a hypothetical. The non-existence of workers with clean drug tests in the real applicant pool doesn't mean that a failing drug test is less of a risk signal in real applicants.

Remember, new hires are not guaranteed to add value - even if they worked for free, there is always a risk that some will reduce value. If a new hire's inability to show up on time holds up a production line, or a new hire's irresponsibility damages equipment, then you're better off not making the hire, even if you desire, in the abstract, to hire more people and expand your business.