Well, per google, which is apparently the only source universally trusted, narcotics are
"a drug or other substance affecting mood or behavior and sold for nonmedical purposes, especially an illegal one."
Words may have a root (like "narcosis") but their definitions are what is commonly acceptable, and often deviate from the derivation. That is why dictionaries constantly need updating

Google's good for a lot of things but I'm not sure it's the best dictionary. The theory I have heard about why cops hijacked the term is because it sounds a lot more spooky
than "tranquilizer" or "sedative" or "mild hallucinogen" or "dissociative." Very effective Drug War lingo. George Orwell would be proud.