Twitch poison and Rageblade

In a few recent games as Twitch i have purchased Guinsoo's Rageblade and noticed that the Guinsoo's Rage will not apply the passive poison. I was wondering if this is a bug, since the poison is an on-hit effect (at least Runaan's Hurricane counts it as one). Could some one clarify whether this is a bug or just how Riot wants it to work. Thanks!