[link|http://www.bartleby.com/68/29/1529.html|Bartleby]:
Kenneth G. Wilson (1923\ufffd). The Columbia Guide to Standard American English. 1993.
could care less, couldn\ufffdt care less
Although many have tried, no one has yet satisfactorily explained how the negative fell out of the older and accepted couldn\ufffdt care less, although logic of course says that could care less makes no sense. Both forms are clich\ufffds, and both are certainly best restricted to the lower Conversational levels and the most Informal of writing. But could care less could also earn you the scorn of logic lovers and clich\ufffd haters alike. Avoid it as Nonstandard.
I agree. It's one of those idioms that people say without thinking about. Rather like "The point is is that ..."
It reminds me of that famous song, sung by Billie Holiday, The Platters, Fred Sanford, and others: "If I couldn't care less" - [link|http://lyrics.rare-lyrics.com/B/Billie-Holiday/If-I-Didn't-Care.html|Jack Lawrence / Milton Ager]. ;-p
Cheers,
Scott.