The speaker already cares so he COULD care less than he already does.
Obviously most people want to say - "I couldn't care less" in order to emphasize how little they care. They end up stating something that is ambiguous, but certainly suggests that they care at least a little even if they intend that meaning.
I cannot believe that the majority of people misinterpret this one: simply put, one cannot care less about something unless one cares for it. If you already do not care, and then somehow "care less," then you have achieved some amount of "negative or less-than-zero" care. How is that possible?
The speaker cared in the past and regrets it now.