It seems like you're talking about what you think ought to happen rather than what actually happens. I'm not saying that social standards are correct in a moral sense but rather that they do objectively exist and control most people's behavior. Maybe the world in which people usually did fight back would be a better world than ours, but it isn't our world.

As for your specific question about the screaming child: the chance that it's scenario A is nearly 100%. Public kidnappings like that do happen but they're very rare. (I think that in practice, people would ignore a woman doing it but sometimes intervene against a man doing it.)