First, it's worth remembering that we don't know what was said, or how it was said. Other than the bald-headed bit, the rest is disputed. So we might all be jumping a shadows a bit.
I do think there is something different between booing in the stands and yelling at someone's face. I suspect most understand that distinction.
It's an interesting question whether abusing an umpire from 50m away in the stands or 5m away to someone's face is the same thing, or if there's a distinction between yelling from your seat in the first row versus moving to the race to yell as someone leaves the arena. I know it's a subtle distinction, but I think those things are different because they affect the target differently, and in respect of abusing someone that's what's important. If I rant and rave in my living room who cares? If I do the same in the stand it might affect those supporters around me, but if I call someone a maggot or a bald-headed [something] to their face then that more profoundly affects that person, and I would know that when I chose to say it.
I know people who view the world differently might think that it's the act itself that should be viewed, no matter where it occurs, not the effect on the person on the receiving end, but I just don't see things that way.
I agree that Nicholls would have heard a lot in his time, but to me that raises the question of why he reacted this way this time, and not the 350 other games this happened? Perhaps what was said (or the way it was said) was worse than usual, perhaps it was arbitrary, or he's suddenly got a thin skin, or perhaps he was emboldened by a new direction from the AFL. I have no idea, but as usual, I'm just don't understand how in the absence of any evidence people can reach for the conclusion it's an issue that resides with the target of the abuse or some sort of AFL overreach.