Some economists would argue that any contracts voluntarily entered into should be enforced. That is what lead them to argue that, if I agree not to redistribute your book, then I should be bound by that agreement. In this view, the copyright law simply codifies the contract that sellers of embodied ideas would wish to bind their buyers to, and so saves on private transaction costs.
What do Boldrin and Levine have to say against the view that they have so aptly summarized? They make an analogy to the ban on slavery contracts, and say that this is "not only a 'morally just' prohibition, as many economists have argued, it is also an economically efficient one."
For starters, I don't see what the moral or economic problem with a slavery contract is supposed to be. Boldrin and Levine just assume the reader will agree that a slavery contract is morally offensive. Their economic case mostly consists in pointing out some disadvantages of slavery-like contracts, without giving due credit to the compensating advantages that lead such contracts to be signed despite their drawbacks.
But suppose you share the widespread view that slavery contracts are just wrong and should not be allowed. Question: Doesn't it have something to do with the fact that a person is ceding everything they will ever have - or at least a high fraction thereof - to another person, AND will have no easy way to back out? Would you have a moral problem with a five-minute slavery contract? What if you could cancel your five-minute slavery contract for a $1 fee?
That's about how onerous the typical copyright is. A contract not to copy the Hero System Fifth Edition core rulebook is at worst a petty inconvenience, not a daily albatross around my neck. And if I did feel oppressed, I could always sell the book, or just burn it, and I'd once again be free as a bird.