The reason for this is that somehow in the last week, I came across an interesting post on this very subject at Dictionary.com in their previously unknown to me section called Word Fact. Here they explain that the expression "I couldn't care less" popped up in British English at the beginning of the twentieth century, but that the variant "I could care less" took root in America in the 1960s. They then go on to point out that the fastidious among us find the American version to be logically flawed. Obviously, if you could care less, then you care more than you might. Here's comedian David Mitchell's rant on the topic.
Nevertheless, the aberrant expression persists. Word Fact goes on to say that some etymologists have attempted to explain it like this:
“I could care less” emerged as a sarcastic variant employing Yiddish humor. They point to the different intonations used in saying “I couldn’t care less” versus “I could care less.” The latter mirrors the intonation of the sarcastic Yiddish-English phrase “I should be so lucky!” where the verb is stressed.
But Word Fact isn't having any of it. It goes on to present my real defense:
The argument of logic falls apart when you consider the fact that both these phrases are idioms. In English, along with other languages, idioms are not required to follow logic, and to point out the lack of logic in one idiom and not all idioms is…illogical.
Which bring us to question, what is an idiom? The Online Etymology Dictionary is very good on this. It tells us that the word entered English in the 1580s and meant a form of speech peculiar to a people or place. It took until the 1620s for it to have our more current meaning of a phrase or expression peculiar to a language. Coming to English through the typical French route, it goes back to the late Latin idioma, which meant "a peculiarity in language", to the Greek idioma. Fowler, of Fowler's Modern English usage, has this to say: "A manifestation of the peculiar" is "the closest possible translation of the Greek word". The root idios we know from words like "idiosyncratic" and means "particular to oneself".
And from the same entry, another quote Fowler:
[G]rammar & idiom are independent categories; being applicable to the same material, they sometimes agree & sometimes disagree about particular specimens of it; the most can be said is that what is idiomatic is far more often grammatical than ungrammatical, but that is worth saying, because grammar & idiom are sometimes treated as incompatibles
I rest my case.
(The signage at the top I found on Jenz Grammer Tips. I think you can guess which side she weighs in on.)