@kickycan,
What they MEAN is "I couldn't care less", but they aren't careful with their speech. It's a clear example of why someone else came up with the phrase, "I know you think you understood what I said, but did you understand that what I said isn't what I meant?"
And if you really want to leave someone puzzled as to whether or not you agree with them, try using the answer, " I couldn't possibly fail not to disagree with you any less." (which is my favorite dissembling to leave for someone else to disassemble.)