It's all very well to say that Jada Pinkett Smith is capable of standing up for herself and doesn't need her husband to wade in and defend her. It's true that she seems to be an independent and capable woman.
But think of something that you really feel sensitive about, maybe something that has made you feel ashamed and depressed even though it's not your fault. Then imagine that someone onstage had cracked a joke about it, in front of a worldwide audience, and the cameras were on you to witness your reaction. Yes, you might have the strength and presence of mind to answer back but it's equally likely that you would just wish you were dead.
Under those circumstances, if my loved one was the target of the joke, I might lash out and give their tormentor a smack in the mouth. I am not saying that I would be right to do it, just that I might do it anyway. No-one condones violence. We all know that it isn't the way to go. But it's not hard to understand why Will Smith reacted like that, if he looked at his wife and saw that she was hurting.
As for the idea that it was all a put-up job for publicity.. Well, if it was, then it worked really well.