With music, they had a limited choices: 1) Keep DRM and deal with Apple but lose the non-Apple market; 2) Deal with a bunch of other DRM formats that don't work with iPods; or 3) dump DRM and be able to sell to everyone (with a side strategy of suing downloaders). For video, there is no dominate hardware manufacture that has a proprietary DRM format -- almost everyone that sells uses a software player of their own design. Ebooks are the one case I don't understand though.