So, they are have to deal with your horse and have to put up with any bad behavior your horse decides to do along the way?
I can not imagine telling my barn owner that they are not allowed to punish my horse when he has one of those very rare moments where he forgets he is a good boy.
I would expect and encourage my BO to do this. My horse would be in a "public" place where I was not available to watch over his actions all of the time. No, I'm not saying beat him, jerk his face, kick him, etc. But smacking him on the shoulder isn't going to hurt him and if you use it sparingly it's an effective form of discipline because your horse will think to himself "oh, no one ever smacks me, I must have done something wrong!!"