I've just finished reading Kinsella's Against Intellectual Property. I strongly encourage anyone interested in the subject to read it. What strikes me is that most of the people I ask about copyright-related matters are willing to accept arguments for IP that they'd reject as absurd wrt to physical property.
Kinsella covers a number of important points -- the consequences of the scarcity justification for property rights, discussed further at Right to Create, deserve more attention in this debate -- but I'm more interested in his claim that one has rights only to protection of the integrity of one's property, not it's value.
Most, I've found, accept Thomas Jefferson's argument that the use, reproduction, or exploitation of another's idea in no way diminishes the integrity of the idea. That is to say, when I use your idea, I take nothing away from the idea itself. This is, in my experience, an uncontroversial position.
Instead, defenses of intellectual property laws focus on the effect such use has on the market value of the idea. Creative works, it is argued, derive their economic value from copyright and would therefore decrease in worth without the protection of a government-issued monopoly. Is this fact adequate justification for copyright law?
It's instructive that this argument is rejected prima facie wrt to physical property. If competition, opinion, or any other legalized force reduces the market value of your fishing boat, you have no grounds for redress, provided that action in no way violates the integrity of your property. If I wage a campaign to convince kids that toy cars are boring, toy car manufacturers have no reasonable claim to damages caused by my reduction of the value of their property. It is, in fact, the integrity of physical property that is protected from harm by law. Events that merely reduce your property's value are outside of its scope.
The same is nearly true regarding IP. The same amount of protection from critcism -- i.e., none -- is provided to creative works. Authors have no right to be free from negative reviews. It's tempting to believe we've stumbled on a sort of equity of ideas. But it's important to realize that it is not a right to the protection of your property's value that is violated when I trespass on your land. In fact, one can easily imagine a scenario in which the trespass results in a net increase in the value of your property. I might staple $100 bills to your trees, for example. Your right to protection from any such act is secured, despite incidental personal benefit. In other words, our property rights are independent of economic considerations.
It's of course true that a consideration of damages may enter into proceedings regarding restitution for damages and that the same would be true wrt economic damages resulting from IP violations, but that says nothing about the justification for the right itself. The answer to the question, "Why does the abused party deserve restitution?" is not simply "Because his property lost value." It's the violation of a more fundamental right that raises the question in the first place.
It's my view that a rational defense of copyright law as it exists today should include justification for the idea that the value of property deserves the same respect and protection as its integrity. At the very least, the burden is on supporters of IP to explain why they afford greater rights to those who would profit from ideas than to owners of tangible goods.