People really say this ("I could care less") to express that they do not care at all. I've seen it happen here on this site. Calling out the sheer absurdity of it, even in a respectful way, is not universally well-received. Unfortunately, I could care less about this, as it sounds very grating to me.
I try to remember that I ain't got no problem with other "illogical" uses of negation and could this one in a similar light, but it's more easily said than done.
I assumed it was an American thing. I've never heard anyone on this side of the Atlantic say it – even though Americanisms are being adopted more by the younger generations who are more influenced by online culture.
No, people who properly speak American English say "I couldn't care less" as well. It's just that there are a lot of people who don't speak the language correctly, and no social will to try to get them to learn to speak it correctly.
> But if you are the kind of person who cries out against this abomination we must warn you that people who go through life expecting informal variant idioms in English to behave logically are setting themselves up for a lifetime of hurt.
Downvotes are likely coming from the fact that this comment is quite off topic / doesn't reply to the substance of the comment you are replying to.
While it's often not helpful for folks to point out that something is a quick internet search away, performing one is usually best before going off topic. Doing so would have resulted in the answer to your question pretty quick.
Do people really say this? Is it exclusive? I've only heard the inverse: "I couldn't care less".
Edit: genuine question. Please explain downvotes!