The philosophical basis of copyright law is a travesty that is not worth going into here. The resultant statist defense of "intellectual property" is heavy handed and infected with cronyism because the philosophical basis is of such poverty. The typical libertarian response is to disavow any right, as such, to protection of intellectual property.
I'd like to suggest this is an extreme response and describe how something akin to copyright is valid in a free market, libertarian society.
The fundamental basis of libertarianism is private property. In this theory there can be no question that the correct and proper owner of work is the creator of that work. (A possible caveat is that all the inputs to the work were already owned by the creator - if you create a sculpture out of my marble, not only is that sculpture mine, I may have recourse against you for using the marble without my permission.) In terms of homesteading, the creator of a work has a greater claim than any subsequent claimant as they are, from the point of view of the creator, latecomers.
The basis of private property is the self-ownership concept. I am well within my rights to offer to sing a sonnet for you, in exchange for a mutually agreed price. Our agreement is called a contract, and it is of no business of any third party what the terms of our contract are. This would appear to be an argument for intellectual property, such that sonnets are works of the mind - and some have argued exactly that - but I think we need to go deeper.
What kind of price might I demand for my performance? In a free market, the answer is clear: anything the market will bare. If I so desire, part of the price I demand can be secrecy. If my audience is not willing to meet my price, they have no right to hear my performance. In a sense, I have prevented reproduction of my performance, not by the skill and grace of my voice (trust me, there isn't any), but by the contractual arrangement under which it was given.
This method continues to function if the intellectual work is provided in a fixed form.. be it written on paper, painted on a canvas, etched into a sculpture, or encoded in a computer program. If I require the purchaser to maintain secrecy, or even just refrain from making copies, then I have a legal recourse if the contract is subsequently broken.
One may object on semantic grounds to the use of the word "purchase" when referring to a contract of this sort. While it is clear that the purchaser of a performance is not buying the performer, it is less clear that the purchaser of a sculpture which comes with restrictions on its use, is indeed "purchasing" the sculpture. If it helps, one may reasonably say that the sculpture isn't being purchased at all - it is only being rented, and comes with a rental agreement that includes some restrictions. This would be a fair characterization.
We can now anticipate what will happen if the contract is violated. As it is absurd for a sucker who buys a rented car to expect to be able to keep it, so too any receiver would be amiss to believe he could keep a copy of our rented sculpture. His grudge at having his copy removed from him would belong to he who sold it to him. As in all property disputes, a claim of title passed from seller to buyer is required to prove legitimacy. Any receiver wanting to keep his copy would present such a title, allowing the trace back to the original contract violation by the ultimate culprit, no matter how many copies were made.
Under such a Liaise-fair copyright system, the costs of enforcing a copyright belong with the owner of the original work. However, just as with all other contracts, the courts are at his disposal to enforce his correct claims.
This is not to imply that such a system would necessarily be successful. It is simply a given of free societies that contracts are voluntary. Upon reading a contract with a copyright clause such as this one, a trader may choose to seek goods from elsewhere. The success or failure of such a system as this one would be decided by the state of the market in which it is attempted. This is as it should be.