it wasn't a punch, it was a slap
personally, I say good on him, Chris rock made a cheap shot for a cheap laugh ( which is expected with Chris rock ) at Will's wife, She took offence to it / was upset by it ( and why shouldn't she, alopecia can be a pretty big thing for people, imho especially women who want to look good with a nice "hair do") Will saw that and dealt with it
I guess Chris rock thought he was pretty safe making the cheap joke on stage at the Oscar's, he wasn't and it had a consequence
Well done Will