All sorts of red herrings here.
Legality of usage in broader society is irrelevant. Just to give one example, there are all sorts of over-the-counter drugs that contain stimulants on the WADA "prohibited list".
"Performance-enhancing" is also irrelevant. Cannabinoids are categorized by WADA as a "substance of abuse" (along with cocaine, heroin, ecstasy, etc.) and are only "prohibited in-competition" (as opposed to, say, anabolic steroids, which are "prohibited at all times")
Richardson is suspended not because of the absurdity of the rule, which is also irrelevant. She's suspended because (as she herself admits) she lacked the discipline and self-control to refrain from consuming a substance that she knew was banned in competition. Elite athletes in drug-tested sports have the responsibility to be meticulous, obsessive even, about everything they put in their bodies.
If there's one thing that I find encouraging for the rest of her career, it's that she's owning her mistake and not trying to shift the blame onto the rule for being stupid.