Google and Amazon's approach has some big potential to blow up in their faces. It's definitely going to create some interesting legal questions. For instance Google's YouTube copyright filter applies equally to private videos. What makes music different? It's a slightly different circumstance of course but Google will have to explain why they think private pirated video is a no-no but private pirated audio is not. I think they'll also have some burden to prove only one user is accessing an account. For example when Google Music invites are flying around what stops me from creating multiple accounts and simply giving the username/password to my friends?