Not 1 month ago I was hit in the face by a 6th dan, Yoshinkan. I had been his uke all weekend, and this was the last day of training. Sensei was demonstrating Shihonage as done from Shumatsu Dosa Ichi. The first movement involves atemi to the uke's face. He began demonstrating it in front of the class, and on the 3rd or 4th demonstration, I'm not sure where my mind was, I forgot to block. Sensei looked me dead in the eye and changed his backfist to an open palm and slapped me right on the mouth and nose, saying "Uke should block here."
I simply put my hand up between his hand and my face and said "Hai, sensei." That was it. I feel in no way that he was abusing me. He hit me firm enough to sting for a couple of minutes, but he didn't jar my teeth loose or anything. I don't feel in any way abused my this. I've been a shodan for two years and test for my nidan in the Spring. It's my responisiblity to rember what to do and when to block. In my opinion this sensei helped me with my training, he did not hamper it. As it is said: "Pain is a good teacher."
I realize there abuse can come about, and a dojo is no place for it. If you think you're being abused, deal with the situation or get out. But don't think you're going to seriously practice a martial art for any length of time and not get hit.