Derek Slater has been explaining why the "you wouldn't steal the shopping cart, so you shouldn't use the copyrighted work without permission" analogy breaks down, even though it is intuitively appealing rhetoric.
"The point of the comparison to shopping carts is to make us think that whatever intuitions apply to one case should apply to the other. So, if our intuitions say that stealing a shopping cart is wrong, using a copyrighted work without permission is also wrong - in both cases, we're taking something away from the rightful owner.
But what about libraries? Libraries provide free access to copyrighted works without any compensation to copyrighted works. Perhaps libraries should be shut down, and copyright holders should be able to license book-lending services under whatever terms they want.
In general, I bet (or, rather, I hope) that our first intuition about libraries is that they're perfectly justifiable and socially beneficial. If DeLong or others want to push us to accept their analogy to shopping carts, they should also be willing to bite the bullet and say that libraries are at best a tolerable though unfortunate historical accident and at worst unsound policy that must be eliminated as soon as possible; alternatively, he ought to have a clever way of distinguishing this case or clarifying his principle. If these principles lead to ostensibly wrong conclusions, we ought reject the intuitions pumped from the shopping cart comparison."
I fear that copyright expansionists do indeed think of libraries as an intolerable and "unfortunate historical accident" and that we are never too far away from public proclamations to the effect that taxpayer monies are being wasted and the free market is getting distorted by an unsound policies supporting libraries "that must be eliminated as soon as possible." Even in the US, though, such an argument would be still seen as a little extreme.
Update: Derek metions Lawrence Solum's analysis of the ecomomics of intellectual property from 2003 and it really is terrific. Required reading for anyone who genuinely wants to understand the difference between tangible and intellectual property.