To some extent, this is because fundamental rights are tricky things. It's not clear where they come from, or how to recognize them. Nonetheless, meaningfull arguments can be made, and, in the end, they are a lot more valuable than shouting matches.
So here are some arguments....
Copyright is rejected by the great philosophers of fundamental rights
John Locke, who formulated the modern western theory of human rights, did not propose any form of 'intellectual property'. This ommission is telling as Locke generally attempted to cover the field of natural rights exhaustively. More impressive is Thomas Jefferson, who declared that "The exclusive right to invention [is] given not of natural right, but for the benefit of society" even as he supported a strictly limited copyright clause in the US constitution. This perspective was written into the constitution and has been the consesnsus of legal scholars ever since.
I am not suggesting that everything a great phlosopher says is automatically correct. Nonetheless, Jefferson's word is often taken as authoritive on such matters. If we reject the views of Locke and Jefferson, we must reconsider whether fundamental rights exist at all. Furthermore, Jefferson gains credibility from the general success of his ideas at establishing a country. Can any of us claim to approach his achievements?
No religion acknowledges copyright
While I have not read every sacred text everywhere, I have read a great expanse of them. Every one of them that touches on the matter encourages the copying and distribution of wisdom (including themselves, of course!) If fundamental rights come from God -- any god -- they don't include copyright.
Copyright opposes the fundamentals of human culture
One of the few true universals of human society is folk art. Every human culture includes folktales and folk music. These are artistic expressions which have evolved as they are copied and modified throughout generations. Their original creaters are lost. They form the foundations of a culture. The very thing that makes us human is that we don't always follow copyright. So how can copyright be a human right?
"Works for Hire"
Copyright law acknowledges such a thing as a "work for hire" -- that if an employee creates something on work time the copyright is assigned directly to the employer, usually a corporation. If copyright were a human right, it would surely reside with the human in question. A truely fundamental right wouldn't be overridden by an employment contract, any more than a corporation can murder its employees.
Copyrights can be violated with no effect whatsoever on the holder
Let us take an extreme case, for neatness' sake. Consider an American musician in 1920, one whose work is played on the radio. The law at that time established finite copyrights, I think around 40 years. Suppose an extraterrestrial race built a giant radio telescope orbitting Vega. In 1956, the song, still under copyright, would reach that telescope and be illegally distributed among millions of alien scientests. However, special relativity forbids any effect of this piracy to reach the artist before 1972 -- 12 years after the copyright expired. It is therefore theoretically impossible for the artist to be harmed by this type of piracy.
Now, you may say that this is an unreallistic hypothetical (though how do you know?). I chose this as an extreme case. If the copiers simply did the deed in a remote village in the himilayas, there would be no practical effect. I chose Vega only so that harm would be totally impossible.
You may also argue that they are performing the deed outside of American juristiction. This blocks enforcement, but fundamental rights are not effected by borders. They persist despite governments. That's what makes them fundamental.
What sort of fundamental right is it that can be violated without effecting the 'victim'? Not any I'm familiar with.
The closest analogy is bloodlines
Many have argued by analogy to physical goods. This analogy is poor, because a physical object cannot be copied. A better metaphor is non-human bloodlines.
Suppose I were to buy a prize-worthy tomato plant. Thousands of person-hours had gone into selecing the finest tomato plants in the world and carefully cross-polinating them. The plant is very useful: its tomatos are excellent and plentiful.
Now, I could take cuttings or seeds from this plant and give them to my friends. My friends would get the benefit of those thousands of pperson hours for free. No law would stop me. It's just the nature of the product. I don't think anyone would argue that I had acted inappropriately (well, except Monsanto, who added DRM to their latest 'rice'. Some people argued that they could offer whatever products they liked however they liked, but I don't think anyone suggested those Chinese peasants shouldn't have been farming in the first place).
Now how is this different from copyright?
Most of history has had no copyrights, and didn't really miss them
Copyright first appeared only a few centuries ago. Innumerable great artists worked in that era. While many wished for a more effective way to make money, none argued that a fundamental right was being violated, not even those who thought very much in those terms, such as Beethoven.
In fact, the internal cultures of those artists tended to encourage copying, limited only by the desire of the copier to retain artistic control of his own work.
Copyright depends on a difference between speech and art
In theory anything anyone ever says is copyrighted. No one actually endorses following this rule. It is generally acknowledged that a conversation can be repeated, outside of primacy concerns. E-mails are repeatedly forwarded. Newsworthy statements are reported. All of these things are vital. It would be utterly unreasonable to ban such them.
And yet, what is the real difference between an anecdote and a short story? Any line is arbitrary. Furthermore, any such line is unmaintainable in the face of conceptual artists. Can four minutes thirteen seconds of silence be copyrighted? John Cage considers it a work of art, yet imagine the horror if no one was ever allowed to perform excerpts from it without Cage's permission!
I don't claim to have covered the field exhaustively, but I think I've made my point. I also have not addressed the question of whether copyight law is good policy. Please keep my aguments in mind in future debates on the topic.
Comments are appreciated.
Feel free to copy this article wherever you like, but please attribue me if at all convinient.