Issue is not Roon, it is Apple, Amazon, Spotify, etc. They don't have an open API that allows Roon to work. Spotify has Spotify connect, Amazon has selectively worked with a few large partners, no idea about Apple.
Who has Amazon partnered with? One of (if not the primary) reasons I will not use the service is there is nothing out there that is capable (other than one of the Amazon "toys") to run their music remotely. Given the consumer bias Amazon promotes, I dont think we'll see the like of Lumin, et.al. with an app to control the service etc.