Copyright is a law we have made, and those who reject it see it as bad and unjustified.
Who said it isn't "a law we have made"?
I surely did not. I would also drop the patronizing "If you want to understand" start.
I know fully well that copyright is not a physical law.
Access to free copies of things someone else created and sells is not a physical law either. It's something some people want.
I (and the law) happen to find the rights of the creators regarding the distribution and sale of their creation, to be more fundamental than the rights of people merely wanting to have those creations.
And, what I set up to do above is not prove that the copyright law is just or correct as it is, but that some arguments against it (those I examine) don't hold much water.
From a principled position it is basically immoral, and from a pragmatic position it is unproven by evidence
It's a law. It cannot be "unproved by evidence", because it does not make an observation to prove or disprove, but declares how some things are to work. Even a totally arbitrary law, like: "Every person wearing red should be jailed" cannot be "unproved by evidence".
As for the "basically immoral" you make it sound like a statement of fact. It is not. Lots of people, including legislators, don't see it as immoral.
If a law is not justified and well-founded we really ought not to have it. Is that not important?
Of course. But it must be proved that it's not justified. I don't see many arguments in your comment. You write, for example:
From the anti position, all the notions of 'stealing', 'losses', 'will of the creator' no longer exist -- they are entirely dependent on accepting the copyright concept. If there is no copyright, there is no stealing, losses, or primacy of creator's will.
The same holds true for everything, even for murder. All the notions of murder being bad and illegal depend on the acceptance of the "life is a right" concept. Which is a societal concept, no much concern in raw nature about killing and eating other animals or even your own species.