It's an illegal substance; it must be bought from illegal merchants; engaging in black market commerce exposes one to the risk of blackmail, coercion &c.

It'd be weird if it weren't disqualifying.

(I think it ought to be 100% legal, but until it is, users are too great a risk)

Well, sure. In itself. But in terms of a security clearance, a use of drugs means you might be susceptible to bribery/ extortion/ blackmail over such use (or someone might helpfully keep you supplied in exchange for information).