What you're saying is how people generally think the prohibited possessor law works, but it's not how it works in practice. It says nothing of drug possession, only gun/ammo possession by someone who uses or addicted to illegal drugs. There is plenty of reasonable doubt that constructive or actual possession of drugs is not accompanied by use, in fact this is the case for many dispensary workers.

>922 g (3) ... who is an unlawful user of or addicted to any controlled substance (as defined in section 102 of the Controlled Substances Act (21 U.S.C. 802));