As it’s used, “I could care less” works as a sarcastic statement much better than “I couldn’t care less” as a literal one. Generally it’s possible for me to care less about some minute difference, but in absolute terms I don’t care enough for it to matter.

That sounds like a poor justification for simply using the wrong phrase. I've never heard it used sarcastically, just incorrectly.

You damn! I felt that slap ;) :P