Mark Twain spells incorrectly for literary effect.
I don't know if you noticed, but you just contradicted yourself. You say that "dis" is a correct spelling of "dis", yet "dis" isn't a word with its own set of correct spellings (that is, "dis" doesn't have a correct spelling).
That notwithstanding, you go on to say that "dis" is the correct spelling of "this" if we're writing a certain dialogue -- isn't it simpler to say that "dis" is an incorrect spelling of "this", but the accent/dialogue/however you wish to call it demands an incorrect spelling? Isn't it easier to say that, if an author writes "dis", he is merely reflecting the incorrect spelling of his subject matter?
(That is, isn't it easier to say that it's
acceptable to say "dis", even though it's an
incorrect spelling.)
I don't know what you mean by "it's decided by us". Do we vote on it?

I'm not being facetious, but I never decided that "tonite" or "donut" weren't totally wrong. I think they're incorrect spellings. Am I wrong? Am I right? How do we decide?