Is every author who wishes to convey certain messages to their audience through narrative also responsible for every single thing his characters say? Character-driven narrative would seem to be at odds with such a view.

I was wondering about that too. But what I mean by "responsibility" is that the ideas presented have a definite form and don't get to evade criticism by being mercurial and shape-shifting. Not sure about art, like fiction. I'm not seeking to prevent authors from being ambiguously provocative, but it's a crappy way to reason.