I could imagine that Amazon is making the Prime app on other devices bad by intention. Because they want that people use their services on their own Fire TV devices. But neither Netflix nor Spotify have their own hardware. And both have already stopped offering their subscriptions over Apple's AppStore. So if they provide an app for Apple TV, why should they have an interest in making the experience less good than on other devices? Instead of just ignoring Apple TV completely?
So personally I still think that those apps would be in a better shape if the developers could just use the same cross-platform code on ATV. Nowadays many apps are basically just dynamic webpages. But as far as I know Apple doesn't allow WebView on ATV. So developers would have to write workarounds or use wrappers to do things on ATV. So to me it seems that tvOS is just not a very developer friendly platform in this regard. But maybe a developer following this might want to comment on this.