He let seth do that to him because if he retaliated/tried to block/ dodge/ deflect seths attack then it'll count as aggression on Seth's part and the seal put by maat will break instantly triggering obliteration of Seth's soul or end his probation grace period which would result in his destruction or shipping him to row ra's boat as part of his og punishment. Basically horus completely surrendered n clung to seth so that his actions would seem like its fully welcomed and not aggression/harming another person as set by maat.
In the midst of their misunderstanding n temporary fallout n horus's meltdown he doesnt stop to put seth first or protect him. He literally saved him
He knew seth has to let our his anger so he just let him do whatever he wants to him