Google provides what they believe to be improved/more reliable services that they can make guarantees about. The open internet is inherently less than that. Besides the paradox about companies owning proprietary technology to key infrastructure that isn't then shared like early internet technology was, one thing is for sure: Chromecast could have made the tradeoff to use 8.8.8.8 a user choice. When in doubt, ask, but with Google, the culture has always been we know better.