This is Irony

And this expression is often criticized by people who don’t understand that it’s ironic.

Irony is when you say the exact opposite of what you mean. (Also when the exact opposite of an expectation happens.)

Probably a lot of folks who say “I could care less” are being imitative, not deliberately ironic, so perhaps the criticism for using the expression isn’t misplaced. 

I prefer not to be ironic, myself.