I would guess that anyone who sings or plays out of tune is not aware of that fact, meaning they don't hear pitch well enough to know. If they knew, they'd surely remediate it, wouldn't they?

On the other hand, you have certain styles of music where that sound appears to be essential to the music, such as Cajun fiddling. I'm sure Michael Doucet could sound smoother if he wanted to, but the music seems to innately demand that raw, primitive, dodgy-intonation sound.

Cheers,
Michael