It doesn't "specifically" say that. If you read the Betamax case, it's clear that the court was looking at the intrinsic qualities of home video taping devices. Time shifting, for example, isn't really an issue with back ups, recorded music, or books. That's not to say that the Betamax case isn't very persuasive when it comes to fair use with other media, but it's not explicit that the holding in that case would apply to other media.