Probably the first kind of automatic alteration or correction of characters entered by the user by means of a keyboard, were “smart quotes”, i.e. the replacement of
" by proper typographic, localized apostrophe and quotation marks. The only candidate I can imagine coming earlier, is capitalization of the first letter in certain cases (still notoriously failing after abbreviation dots).
Soft keyboards today, especially on mobile devices with touch screens, do a lot more of that (and I do not mean Asian IDEs). For English and related languages, they may insert an apostrophe into a contraction entered without one (which is convenient, because that key is on a different shift level), e.g.
isnt to “isn’t”, but at least iOS chooses “isn't” instead. They also don’t seem to do much “curlysation” elsewhere. The standard iOS keyboard at least gives access to the proper marks, Android does not.
So what could be a justified design rationale to make it harder for users to use more pleasing characters?
It is certainly not encoding or font issues. I know some users claim to prefer straight variants in some (sans-serif) typefaces, but I am looking for reasons other than typeface preferences.