Why does the Gm. say the 'ukimpta' of the father going to galus is that he was teaching his son a second umnus which is not a mitzvah. Why couldn't the GM. say poshut that the father hit his son with anger while learning which l'chorah is not a mitzva?
Yitzchak Silverman, Boro Park USA
Tosfos (DH v'Ha) says that the Gemara understood that the Mishnah is telling us some Chidush which applies specifically to a case of a father and son. If the Mishnah would be dealing with a case where the father hit the son in anger, this would equally apply to any person who hit another when he had no right to do so.
See also Si'ach Yitzchak and Aruch l'Ner.