Here’s an interesting discussion about the phrase “I could/couldn’t care less.” The debate is over what the phrase “should” be: in either case, it’s used to say that a person just doesn’t care, but the internal logic of the phrase “I could care less” might seem to ask for the response “Really? You could? Then you must care a bit about it, right?”
That’s not what speakers mean by it, though. The phrase is a good illustration of the fact that language forms do not evolve according to a literate kind of logic. The article goes on to describe other linguistic peeves–the difference between irritate and aggravate, for instance. That’s not quite the context I’d use to interpret this. A better comparison would be to double negatives: saying “Don’t you NEVER get near my daughter!” might “logically” ask for a response like “Really? Well, how near can I get then?” It would take a willful fool, though, to think that the speaker is actually giving permission to take her on a date. A double (or triple) negative clearly wants to repeat a demand, not deny it.
These are the pet peeves of prescriptivists, but language is not a logical creation, however much we might try to demand that it accord to an abstract kind of intellectual logic. If we did, there’d be no such thing as sarcasm, and no two/to/too words would sound the same for fear of confusion. Language evolved as a social, oral, and gestural skill that demands attention to context. The demand for logic is a bit like asking a metaphor to make sense–saying that “the sky weeps” makes no logical sense at all, but again, most listeners wouldn’t look for enormous tear ducts at the corners of clouds.