How much less? The phrase, when used properly, is meant to convey the idea that the speaker is completely and utterly uninterested in the topic being discussed. In the garbled version the message of absolute disinterest is lost.
"I couldn't care less" came from the British in the early 1900s, but around the 1960s, Americans inverted it to "I could care less." No one is sure why. Perhaps it was an attempt at sarcasm that's been lost because what you're really saying is while you don't care about the topic, there are others about which you care even less [sources: Washington State University, Qunion].