> Please try to use the principle of charitable reading when using the Internet
The most charitable interpretation is that they meant to write that they didn't give a shit, regardless of what their interlocutor is going to say. Thus, they couldn't care less, as in absolute impossibility of caring any less. I understand where you are coming from, and it makes sense, but I absolutely guarantee that is not what people intend to express when they say "I could care less".
Anyway, shittier idioms exist that are probably born this exact way. Irregardless is no longer detected by spell checkers as a mistake, so hey, english language doesn't necessarily need to make grammatical or etymological sense.