Maybe you'd understand the concept better if it was your wife, mother, or daughter that was spat upon.
If my wife, mother, or daughter was a traitor like jane fonda, I'd spit on her myself. O'reilley was whining about the guy spitting on fonda. I think he was scared that somebody might do it to him.
Spitting has been used around the globe and throughout history to express disdain and contempt. It is even in the bible:
Yeshua our kinsman-redeemer was subjected to the humiliation of being beaten and spat upon by those who mockingly said to him "prophesy to us Messiah, who hit you?" The people were awaiting a Messiah whom they thought would redeem them from Roman oppression, not from their slavery to sin. He was treated with the gesture of contempt reserved for one who was unwilling to redeem.(Ph. 2:7-8).
Jane Fonda evokes visceral reactions from many of us, who recall vividly what she did during the Vietnam era. If you understand the emotional aspect of this, you will understand why many of us are not upset by the rather innocuous gesture of spitting on someone whom we consider to be a traitor.