Concerning your question about Faulkner's "Barn Burning," first, I've never heard of italics being equated with punctuation, of italics being a form of punctuation.
In "Barn Burning," italics are used in place of punctuation. Specifically, they serve the purpose quotation marks usually serve. The italics simply signify Sarty's thoughts. Italics are used to mark dialogue, albeit dialogue within Sarty's mind.
For example, here's a passage from the opening paragraph--italics included:
He [Sarty] could not see the table where the Justice sat and before which his father and his father's enemy (our enemy he thought in that despair; ourn! mine and his both! He's my father!) stood, but he could hear them, the two of them that is, because his father had said no word yet:...
The italics are just used as quotation marks.