Kevin Drum has a provocative thought:
I am, as always, speaking only for myself, but I think this is too cramped. The Constitution says that the purpose of patents and copyright is to "promote the Progress of Science and useful Arts," but the fact that the Constitution says this doesn't mean it's the only reason to grant patents and copyrights. There's another reason too: because creators have a moral right to profit from their works. In real life, pretty much everyone acts as if they believe this, and I suspect that for most of us it's the real underpinning of our support for IP law.
I don't think this works, though as a broke-ass writer I would much like it to be true. The act of writing, or making music, or whatever, is not an intrinsically worthy undertaking. Suppose I write some book and put it in a cabinet in my basement. Do I deserve to profit from that, simply from the act of creating it? If Hitler yet lived would he have a moral claim to monopoly rents from Mein Kampf? Does everyone who borrows a book from a friend, or checks one out from the library, have a moral obligation to pay the author? This is absurd.
I do think, however, that some moral principle might be salvaged. The readers are what is missing from the equation. No one deserves to profit from something that is not read. Here's a try: anyone who consumes some content, and likes it, has a moral obligation to compensate the creator in some way. This is to say that abject illegal downloading is morally wrong. It could take several forms, including just passing the word that it was a great work and you enjoyed it, especially if you are famous yourself. Publicity, especially for relative unknowns, is often more valuable to the creator than cash.
Can anyone come up with a better principle?