But just because people think all law is about morality doesn't make it so. It's the job of a philosopher to question people's assumptions and identify errors in them. This looks like a classic case of hasty generalization. Some laws are about morality. Other laws might not be.
I think copyright belongs in the non-moral category. You have not offered any reason to disagree with my statement other than a vague "But people in the 18th century believed otherwise!" Well, I'm not disputing your historical facts. The fact is that people in the 18th century believed a lot of things. Some were true, some were not.
Again, this is at odds with your initial statement. And again, for some quite large and significant aspects of copyright.
This confusion was no doubt intended by those who coined the term "moral rights". But they are not the same thing. Copyright law is not needed to enforce moral rights, as you can very well see from the fact that we do acknowledge the creators of works that predate copyright by hundreds or thousands of years. We can freely copy Homer's works. And we do. Be we don't remove his name from them.
Reegardless of how you feel about that scope of law (and I'm not declaring views one way or the other), the point remains that your initial assertion does not square with the stated foundations of law, in at least part. Regardless of how separable or inseparable those moral rights are from any other.
If the HN hivemind wants to declare its support for the counterfactual, so be it.