Copyright laws are written to protect the author (or translator, artist, etc, but I'll stick to authors for my discussion). Without copyright laws, it would be easy for a very skillful author to write a wonderful book, but for some wealthy publisher to come along and take their book and publish it for their own profit. The author then goes un-recognized and more importantly, un-paid.
Copyright, just like patent law, is designed to protect "the little guy," and to make sure that someone who spends a year writing a novel, or researching a particular area to write about it, or spends decades translating the Bible to their own language, etc, has the opportunity to be rewarded for their work.
Clearly, copyright laws can be used as tools for greed, too, but I don't think that invalidates the reason for the laws, any more than it invalidates the need for banks or supermarkets. Just because something can be abused does not make the thing inherently evil.
If a Christian author does not approve of copyright laws, he is free to release his book to the public domain (or with some other non-traditional copyright, as has become popular the last few decades). And in fact, if his purpose for writing a book is to enlighten others, and not to earn a royalty, he may well feel the desire to do this. This practice is especially common in certain areas of software development.
But I don't think a strong Biblical argument can be made that copyright is inherently immoral or unethical, nor that Christians ought to copyright their works. I think we can glean from scripture, though, that Christians are bound by copyright laws. See Romans 13:1-2.
To me, copyright is an important way to protect the rights of the "little guy." It clearly can be abused, but practically anything good can be abused, so the possibility of abuse is not a good enough reason to do away with something.
Because I view copyright as an important way to protect the rights of the otherwise powerless, I would could not in good conscience fight to abolish copyright law, as I would see it as violating the principles laid out in scripture, of caring for "the least of these," etc. For me, this would be a sin.
However, I cannot say that a Christian who feels copyright is wrong would be sinning to fight to abolish copyright. I would ask a Christian who feels it's wrong to explain why, based on scripture, they think it's wrong. If their reasons are purely selfish ("I want to download MP3s"), I would say they are behaving sinfully, and I would urge them re-examine their motives.
If, on the other hand, their motives are selfless ("I feel copyright victimizes people"), then, while I might disagree with their conclusion, I would see their motives as pure, and I would not consider it a sin.
Personally, I would support certain copyright reforms... as I think certain aspects of copyright law are in place only to protect large corporations--and that is not why I support copyright law. But doing away with copyright law entirely, to me, is throwing the baby out with the bathwater.