You're WRONG, WRONG, WRONG!!!!
If you COULD care LESS, then that means you ARE capable of caring less. The whole point of saying 'I COULDN'T care less' is to underscore the fact that you COULD NOT CARE LESS than you already do at that very moment!!!
Think of the other extreme, CARING instead of NOT CARING: I COULD care more, or I CARE NOT care more? If you choose the latter then YOU WERE WRONG AND AGREE WITH ME !
Hold on to your horses, man.
Although your point is valid, it is not the only correct answer.
People say "I couldn't care less" and yet they could. For instance they could not even be 'talking' about the subject, or being on a thread posting about it. So when they say "I couldn't care less", what they really mean is "I could care less".