The next time you feel compelled to "correct" someone for saying "I could care less," ask yourself these two questions: "Is there any ambiguity of meaning?" "Am I adding anything to this conversation by pointing this out?" You'll find the answers are almost always "no" and "no."
Yeah, "I could care less" has a literal meaning opposite the one intended. And you know what? I could care less. It's an idiomatic expression. In a similar vein are "head over heels" and "cheap at half the price." English is full of them.
People started saying these silly things because they rolled off the tongue a bit easier. Ask any francophone, that's a big motivator for otherwise nonsensical lingual oddities. Or, if you want to see for yourself, check out the staggering number of irregular English verbs.
Why have such nonsensical expressions endured? The garbled expressions are unlikely to see literal use anyway. When was the last time you wanted to indicate that your head was actually above your heels, or that that necklace would be pretty well cheap if only it were half as expensive?
Rather than get in a tizzy over "I could care less," why not spend your time contemplating more pleasant things, like beautiful women? It's better for your health, don't you know.