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.
Yes, that's why modern literature and media dealing with diverse opinions are terrible now.
You are expected to caricature and refute people saying "bad" opinions in the work itself since otherwise the reader could believe in those opinions. Leaving something open to interpretation is tantamount to endorsement.
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.