Using Linux means DRM protected content either plays in terrible quality or in RakutenTV’s case not at all. Netflix is limited to 720p with low bitrate and Amazon limits to ~540p.
Changing user agent doesn’t work because it’s the DRM who decides whether the OS is supported.
Linux users have to decide between low quality legal streaming services, or piracy with high quality. It’s not a difficult decision for me and my giant HDD.
Edit: I forgot the third option: streaming sticks (Roku, FireTV).