Consider Netflix; most set top boxes are *nix based, Wii and PS3 both *nix based. They have Netflix players available so implementing Silverlight's DRM on *nix is obviously possible and already done. Why no Netflix players for general purpose non-Win/osX? Microsoft chooses not to license the DRM component for use on other general purpose OS even though they support the Moonlight port of Silverlight to those platforms and there is no shortage of FOSS developers who'll make it work with Moonlight/Netflix.

I'm guessing the Kindle is the same; DRM framework intentionally withheld from implementation outside of the device and few blessed OS.

With DVD and PDFs.. both services can get stuffed as long as they choose not to accept my OS choice and money.