Because if we disagree, then one, the other, or both, are wrong.

There is (both logically and empirically) only one moral law, and it is the basis for all law from the common law to international law : reciprocity.

The only question is, given the demographics, economy, norms, and institutions, and traditions, whether the current order provides reciprocity, free riding, parasitism, predation, or all of the above.

The only reason we can ask this question today is because we have gained sufficient wealth that we desire to specialize in self fulfillment rather than cooperative survival, and with our specialization, form many more smaller more specialized groups. But this is impossible under large diverse governments.

The optimum solution is to divide into groups with shared moral biases (and pay the price and gain the reward for doing so).

It is trivial to teach morality. The silver rule: do not unto others as you would not want done unto you, and do unto others ONLY what they wish done to them. The golden rule merely amplifies the silver rule: do unto others as you would have done unto you – but do not expect reciprocity. You are merely trying to encourage them to prefer cooperating with you rather than someone else more rewarding. The value of the golden rule is that exhaustion of attempts at cooperation tends to (in all cases) produce more cooperation than any other strategy.

That’s it.That’s all there is. The rest is just techicalities of achieving some form of voluntary cooperation in any set of circumstances.