The point is that a crowd doesn't always have a right answer... popular opinion can be wrong
By that logic, all the laws, regulations and norms would possibly be meaningless. So if we were to ignore those, we all would descend into anarchy.
A morality by plurality is arbitrary and non-universal
In fact, a majority of popular support is a necessary (may not be sufficient) condition for morality of a society. Something is not considered moral if it is not supported by most of the members.
society A vs society B
irrelevant. Morality based laws in a society generally consider morality only of that society. That's why we have the notion of sovereignty.