They, too, can read and write, of course, but their  solecisms,   barbarisms,  and  vulgarisms  call for rebuke, and what better rebuke than to treat them as if they did not belong at all'to condemn them figuratively to the outer darkness of the unlettered?
They can read and write, but they aren't exactly polished, so we treat them as poorly as those who can't read and write.