Well, it's interesting what YouTube is doing. Their policy is simply there to cover themselves legally and shift the blame to the users. However this opt-out policy allows them to host the videos and continue making money (or at least getting traffic) from them until the copyright owner calls to complain.
In YouTube's defense, it's nearly impossible to verify the copyright of user supplied content and since YouTube is simply offering to provide hosting of user supplied content it shouldn't be liable for copyright infringements just as your ISP is not liable if you decide to post illegal material on their servers. They will remove the material (and possibly terminate your account) if someone reports the violation.
In YouTube's defense, it's nearly impossible to verify the copyright of user supplied content and since YouTube is simply offering to provide hosting of user supplied content it shouldn't be liable for copyright infringements just as your ISP is not liable if you decide to post illegal material on their servers. They will remove the material (and possibly terminate your account) if someone reports the violation.