Buy one use anywhere/ run anywhere is idealistic. It would be awesome if that is the case but users will settle for less. In Practice users like semi universality. For music a spotify subscription which works in most places is practical. As for movies – its a stream or a rental. Nobody is going to hold on to it. (Its really really stupid to pay 10+$’s to buy a movie which you are probably not going to watch again and if you do it will not work for the reasons you outlined)

Books is where DRM still rears its ugly head – because book are bought. Amazons solution is to have an app for it almost everywhere.

It is not clean but it is a solution which is better than one of their competitors where you can only read a book on one os.

Yes a company like Amazon would love to be only in the services game. But in the current state of the market where friction free user experience is key, they need to own the entire ecosystem (ie hardware) also. Google also had to get from software into services for the exact same reason.