English speakers do English a whole lot better than they analyse English.
I could care less! you might say sometime in disgust. You might just as easily have said I couldn't care less and meant the same thing! How can this be? When taken literally, the phrase I could care less means "I care more than I might," rather than "I don't care at all." But the beauty of sarcasm is that it can turn meanings on their head, thus allowing could care less to work as an equivalent for couldn't care less. Because of its sarcasm, could care less is more informal than its negative counterpart and may be open to misinterpretation when used in writing. 1
The phrases cannot but and can but present a similar case of a positive and a negative meaning the same thing. For more on this, see cannot