Language and how to use it

I'm reading a bunch of American stories at the moment and two of them have used the same phrase 'he could care less' to mean 'he couldn't care less'. Now I've heard this said myself, in Texas, and while I have no objections to it in dialogue (or even dialog) as a common spoken expression, it just drives me wild to see it used in narrative. What does it mean? It means the character cared about something because it says he could have cared less. But the meaning the writer obviously wants us to take from the statement is that the character COULD NOT have cared less, ie that he felt nothing about the situation; it didn't concern him.

This kind of language creep is downright stupid. People say, every day, things that make no sense and using them as speech in the mouths of your characters is fine and often meaningful. But using a slang statement as part of your description of a character is just not good enough, if it conveys the opposite meaning to the one you wish to express. When I see this kind of laziness the story goes on the reject pile. If the writer can't be bothered to sort out the difference between a cultural expression that has become commonplace, even if it's nonsense, and a true description of a character's state of mind, I don't see why I should be bothered to work it out either.