I don't do drugs; I barely even drink alcohol. But, I think drug testing is a violation of privacy and none of a company's business. This isn't 1950, nobody cares if you're doing drugs on your personal time as long as you're sober for work.
Yes, you need to drug test for clearance. But I'm saying that's a job I'm not applying to unless I have no other choice. It goes straight to the bottom of the pile for me.
My understand is that it comes down how likely it is that the employee could be bought/blackmailed/corrupted. A person with a drug (or gambling) habit could be blackmailed or might need more money to fuel their addiction, making them susceptible to a foreign power buying their support.
Same reason for homosexuality in the past when it was illegal/scandalous.
I'd quibble though that this overlooks that quite a lot (possibly even most) drug users are able to lead productive lives without having to resort to illegal activity to fund their drug use.
The other thing this doesn't cover is that there are many alcoholics that fall into the same trap. Though there again, most alcoholics are able to lead productive lives despite their addiction.
And through personal experience, I unfortunately know quite a few alcoholics that work in defense with various levels of clearance.
Yes, you need to drug test for clearance. But I'm saying that's a job I'm not applying to unless I have no other choice. It goes straight to the bottom of the pile for me.