But if the government can decide what rights there are, then anything they do is morally correct, no? Unless you’re going to hold the government to a higher moral standard than themselves, in which case the government doesn’t actually grant rights; it can only protect or violate them. If we have a higher moral standard than the law, then human rights do not come from the government, they are defined by whatever that higher standard is.
I think the Nazis were an insane and utterly contemptible political party that destroyed a struggling nation to slake their own thirst for power. But if the government decides what rights there are, then they can simply legislate out of existence the rights of anyone under their jurisdiction. Thus, anything the government does to them is justified.
I would argue that what rights there are is inherently a moral argument. “Murder is not a right” is a moral statement, for example. The government doesn’t change what rights it thinks there are without some kind of moral basis for it. Even if it’s primarily done in the legal sense, we still generally act in the legal system based on a system of morality. Another example: “Compelling people to testify against themselves is wrong.” It would be really useful for the state if they could do that, but legally speaking, the US recognizes that there is a right against self-incrimination.
Laws are written because someone, somewhere, found a moral fault in the law. It’s just that some people believe that the only morality is power, and thus anything they do is justified. That’s why we have the Bill of Rights: it’s meant to stop people from simply saying “the government needs this power so we’re going to give it that power.” It isn’t about creating rights, it’s about recognizing and protecting rights that have existed all along.