Sounds sort of like the lack of Youtube on Windows Phone.
Publishing YouTube app on Windows Phone requires Google to put extra effort into developing an app for a completely new platform which I assume requires lot of engineering effort.
Selling Chromecast on Amazon probably does not cost Amazon any extra engineering effort. So, I do not believe that the two situations are equivalent.
It's fairly straightforward as these things go, but to suggest it doesn't require anything is flat out wrong.