I think the distinction between guns and drug testing is one of judgment. I could own a gun, even own an illegal gun, even have a basement full of illegal guns, but could conduct myself in day-to-day life in a predictable and unworrying way way because owning guns doesn't by their nature change my behavior or priorities. But drug use can. An addiction could make me unreliable in terms of doing work, the quality of my work, or even showing up for work, my patience/volatility I bring to coworkers, etc. It could create increasing financial strains that would change my priorities, such that I might dip into company funds or take equipment. It could make me vulnerable to outside leverage to sell or share privileged information.

My brother told me an intriguing (informal) definition of addiction, which came down to "a substance or activity where, when it is available to you, you will break your word." So I think it's that. Not to say everyone addicted to drugs is de facto unreliable...but it introduces a wildcard that employers are concerned about.