I have been hit to learn a technique. I didn't get off the line or block effectiveley. But I have never been slapped.
IMHO, especially in the white western culture, a slap is very insulting and humiliating. I would that instuctors would take into account not only the cultrue they come from, but also the cultrue they are teaching.
While there may be explanations and rationalizations, there is never an excsue for bad manners.