Yes, they can. AFAIK Missouri is the only state where it’s illegal for employers to punish their workers for using legal drugs (alcohol & tobacco) when not at work. Plus IME most employee manuals use terms like “any substance illegal under state or federal law” instead of listing specific drugs.
California: It’s California, man. Will you open up? I got the stuff with me.
Nevada: Who?
California: California, man. Open up.
Nevada: California?
California: Yeah, California.
Nevada: California’s not here.
and scene!