If I was in a position to offer real help and was the only party available to do so, I think I would need to do drop the camera. But it's not that simple in cases of mob activity or the presence of perhaps too many potential helpers. It's case by case.
And it's further complicated by issues where the photographer's presence influences the event, such as implicitly encouraging subjects to behave in exaggerated ways.
For me the most disturbing part of the domestic violence shot is the reflection of the photographer looking on dispassionately. I didn't look at the others, the world is unpleasant enough without that stuff.
I tend to agree; however, it seemed a bit strange to me that the snapper was already sort of in their relationship - I find these odd times and even odder people.