I'm merely pointing out that when we provide information to students we should not qualify it with patently false pet notions.
The "double negative" is one of these.
I accept that English learners need to start out with a pretty arbitrary "rules-bound" form of English. But at some point, they are going to encounter real honest-to-God native English speakers who say things like "I ain't doin' nothin'."
They need to know that when someone says "I ain't doin' nothin'," it is not a sly logic game by the speaker who cleverly really means "I'm doing something."
The so-called "double negative" is used here for negative emphasis.
As it has for hundreds of years in our language despite the "tut-tutting" of "grammarians" who can't quite seem to accept that language doesn't work like mathematics.