Why discuss rational metaphor in ʿilm al-maʿānī rather than exclusively in ʿilm al-bayān? The source's answer centers on the predicative state of an utterance.
How an expression attributes an action is one of the structural and contextual conditions examined in ʿilm al-maʿānī. Figurative use of an individual word is a distinct issue; the relation between a verb and what it is assigned to concerns how a complete utterance is formed and understood.
This division helps readers tell whether the problem lies in a word's meaning or in the ascription of a meaning.