From the Constitution of the United States of America, Article I, Section 8:
To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries
This is one of the powers delegated to Congress. Now, with the Sonny Bono Copyright Term Extension Act, the term of a copyright is the life of the author plus 70 years or 95 years for a corporate owned work. I may be misunderstanding our forefathers, but 70 years after the death of the author seems a bit more than a "limited Time". Originally, copyrights were 14 years plus 14 more if extended. This seems like long enough to make money off of a work, especially considering that, for example, most books are out of print within five years.
The argument I most often hear as to why copyright should be indefinite is that it is property just like any other property. If I have a physical object, like jewelry, for example, I can pass it on to my heirs. Presumably, I worked at my job, earned money, bought the jewelry, and passed it on. Authors, the argument goes, work at their job, create some property (the Work) and should be able to pass it on to their heirs. It all sounds logical except for one thing. It is impossible for me to give a piece of jewelry to my heirs and anyone else at the same time. So-called intellectural property can be given away and retained. In fact, the more it is shared, the better off everyone is. Property laws that prevent stealing of physical property are necessary because taking a physical object deprives the original owner of the property. Laws that preveent the stealing of intellectual property are necessary so that authors and other creators can earn some profit from their work to compensate them for their time. While the supply of the physical object is limited, the supply of intellectual property is essentially infinite as most can be copied very cheaply. The shortage that leads to a higher price is artificially created. Therefore intellectual property and physical property are not the same. So analogies comparing the two are not necessarily valid.
While all laws restrict behavior somewhat, laws against stealing physical property are a balance. I can't take your objects without depriving you of them. The law only restricts my behavior insofar as it would directly affect you. Intellectual property is not like that. There is a potential revenue stream, but you can't "steal" that. You can make a copy of the work and distribute it. This does not deprive the author of his property. Copyright laws do, however, fence off ideas so that other people can't use them. Before copyright, anyone could, for example, write about anything. Now, if someone else writes something, you can't write about it anymore. Admittedly, a lot of ideas wouldn't have been thought of if not for the original author but some would have. In addition, some ideas creep into our culture. Take the song "Happy Birthday." It is traditional in America to sing this at birthday parties. It is a part of our culture. The song is under copyright, though, and if you go to a restaurant, they have to sing something else or pay the author. When the author dies, they will have to pay the author's heirs. Actually, I don't know of the current owners of the copyright are the authors.
I just want to clarify that I understand that copyright law protects the expression of an idea. However, it also restricts derivative works, which does effectively protect certain ideas.
Aside from the simple fact that the language of the Constitution is clear, the original intent of patents and copyright was to enrich society by increasing the common pool of works of art and science. This is not accomplished by walling off ideas and stories and songs. It is done by encouraging the creation of more and more works. By granting a limited copyright, the author can earn his keep. By expiring the copyright, you enrich the commons and you encourage authors to keep creating.
The argument that I find most mind-boggling is the one that an author's heirs are entitled to copyright. I see absolutely no benefit to society to granting the children and grand children of an author a copyright. The author is dead and presumably won't be creating anymore works. There is no reason to try to give him more incentive to work.
A reasonable counter-argument to the above is that, for example, the spouse of an author may have worked to support an author who created a work and then died immediately. Should the wife get some compensation for the support she gave the author? This is one of the reasons why I support a fixed length copyright (say, 14 years) that can be passed on to heirs but when it expires, it's gone.
Another argument is that it costs nothing to grant copyright. This is absolutely false. Copyrights do have a cost. They create an artificial shortage of a good and run the price up. A copy of text book would cost a fraction of the original cost of the book. Now, for a while, this is a good thing as it allows an author to make money off their work. But eventually, this artificial shortage and restriction on copying should be eliminated. Again, this differs from, say, an antique chair. I can't buy a chair and then make copies for all my friends.
I would also like to point out that no author creates in a vacuum. Every work of original art is influenced by the author's past, including books he's read, television shows he's watched, movies he's seen, paintings he's seen, etc... So really, any work created is an extension of what's gone before. Consider the end of copyright the payback to this common pool of works for the inspiration it gave the author. If someone were to come up with a truly original work that was based on nothing before it, I'd be willing to grant an exception to the copyright term. It will never happen, though, and if it did, no one would understand it anyway.
Copyright law has a concept of fair use. While copyright restricts the use of a work except as allowed by the license, the law has provided for a means for individuals to make limited use for such purposes as parody, review, or criticism and personal copies. This is good because it encourages the free exchange of ideas and discussion about works. Unfortunately, the DMCA has now made it illegal to bypass any technological copy protection scheme even if it interferes with fair use. So you may still have the right to show a still from a movie in a movie review, but if you had to crack the DVD encryption to get it, you can go to jail for that. Not only is this a blow to fair use, but it's a criminal statute instead of a civil statute. For violations of copyright, you can be sued and must pay damages. For violations of the DMCA, even if they are not copyright violations, you can go to jail. Not only that, it includes provisions that require carriers, such as web hosts, to pull any material that they receive a complaint about before any lawsuit or court case. Thus, I can have your web page pulled by making a DMCA complaint to your ISP on an official-looking letterhead. Then you have to go and prove you weren't violating it. Great, eh?
Here is an article at FindLaw discussing the lack of merits of the Sonny Bono Copyright Extension Act. It also discusses a court case that is now before the Supreme Court that could limit the power of Congress to continue extending the copyright term. Hopefully the Supreme Court will rule in favor of the plaintiff and save us from infinite copyright terms in twenty year chunks.