I mostly learned English from the computer and books, and I could never understand how it is even possible to write "it's" when you mean "its", or vice versa. They are simply completely different words!
Then I went to my first multi-day conference abroad, and after three days of speaking mostly English, I caught myself doing the same mistake. I guess it's easy when you think in sounds, and not in letters.
@deshipu I think you're right, it's mostly native and more fluent writers that get this wrong and likely, as you say, in proportion to the amount they speak/listen rather than read/write.
@edavies I don't think it actually matters whether they are consistent or "logical" or not — spelling is something you learn by use anyways. Contractions are ambiguous like all other abbreviations — that's the whole idea, you make it shorter but still understandable in context. English is a low-context language, so it's not that bad.
I'm just so annoyed by those mistakes popping up in newspapers, books, articles, etc. — it just shows that the author didn't even bother with proofreading.