If the complaint is that you can't find enough workers because too many are failing drug tests[1], then your hypothetical of having "two job applicants" (for an implicit one spot to be filled) is already moot.
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.
[1] https://www.nytimes.com/2017/07/24/business/economy/drug-tes...