Orthography in service to sentiment

I'm reading this relatively unwonderful novel with a first-person narrator whose internal and external voices differ markedly, a difference that becomes obvious only when she transcribes conversation. In narrating her own story, she uses fairly generic language and standard orthography, but in dialogue, she employs a backwoods dialect, thus:

Narrative voice: I told him it didn't matter.
Dialogue voice: "Hit weren't no nevermind," I said.

I suppose it's possible that the book is trying to make some clever point about the difference between a character's internal and external voices, but if so, it's not working. I've been wracking my brain to come up with an example of this kind of distinction in literature, but the best I can do is a movie, The Piano, in which Holly Hunter's internal monologue differs markedly from her external speech--primarily because she doesn't speak at all. But a first-person narrator who writes "it" uniformly until she's conveying her own speech and then switches to "hit"? That's just wrong.

I suppose this could be related to the common convention of nineteenth-century sentimental fiction, in which the female love interest must speak in standard English while women who speak in any discernible dialect tend to be whores, harridans, or comic relief. I'm thinking, for instance, of Edward Eggleston's The Hoosier Schoolmaster, in which the chief female character grows up in darkest Indiana (if you can imagine a time when Indiana was the wild frontier) surrounded by people speaking broad backwoods dialect but who somehow manages to enunciate as clearly as Katie Couric, primarily so that she can be a suitable wife for the schoolmaster of the title. She's also a stellar speller, a fact that becomes clear during the suspenseful spelling bee that plays an important part in the plot. She would never write "hit" for "it," even if she pronounced it that way.

Fiction has come a long way since The Hoosier Schoolmaster, but the tendency to sacrifice narrative consistency and character development in service to sentiment lingers on. I stuck with the current book for nearly 100 pages, but now I'm giving up. "Hit" just sticks in my craw.