Cory Doctorow lists a few reasons why authors should allow sharing of their books and includes the ethical argument:
the ethical reason is that the alternative is that we chide, criminalize, sue, damn our readers for doing what readers have always done, which is sharing books they love—only now they’re doing it electronically. You know, there’s no solution that arises from telling people to stop using computers in the way that computers were intended to be used. They’re copying machines. So telling the audience for art, telling 70 million American file-sharers that they’re all crooks, and none of them have the right to due process, none of them have the right to privacy, we need to wire-tap all of them, we need to shut down their network connections without notice in order to preserve the anti-copying business model: that’s a deeply unethical position. It puts us in a world in which we are criminalizing average people for participating in their culture.
Talking about ethics makes some feel uncomfortable. However, that discomfort is better once prodded (thus possibly awakened) rather than ignored. Thanks to all the Doctorows out there willing to speak to the ethics of sharing books.