why??

> this particular error

It isn't an error, unless you think language has to be perfectly regular and logical with no warts. That's a fantasy. Language develops organically and as such we should expect contradictions and illogical constructions to arise from time to time.

Everyone knows what "could care less" means. There's no external, objective standard by which you can say it's "incorrect". Stop worrying about it.

An example of this type of inversion that has become widespread: in French, "pas" means "not", but originally meant "step", and came into its current meaning via (roughly) the following sequence:

"Je ne marche" (I don't walk) -> "Je ne marche pas" (something like: I don't walk [even a single] step) -> "Je marche pas" (over time, "ne" became dropped in informal speech, and "pas" carries the negative meaning.)