Because I just say "Ow."
I see a lot of horses get smacked in the face over nipping. Some do it to be naughty, but some do it just because they're mouthy... then I see the horses that are getting a nice scratch and they start to "groom" you back, but some people, as soon as they feel those teeth or the lips wiggling, they'll backhand that horse's muzzle without missing a beat. I was always taught to never strike a horse in the face. I'll pinch their lips slightly as a warning if they are play-nipping, and that seems to work.
Anyway, what do you guys do?
If they hint as if they are going to bit, I usually head them off with a stern verbal warning. If a nip or bite actually comes out, they will immediately get one punch or slap on the horse. And hard. I don't fool around. But I do the punishment once and immediately, and then I go back to what I was doing as if it did not happen.
It is not abuse, it is discipline and I think it is just outright dangerous to allow a horse to bite or nip under any circumstances.
Needless to say, none of my horses bite. They know better because they know who the head of the herd is (ME).