I don't really see the point of changing the DNS settings to watch YouTube, either way Google will know what you're watching. I know the Chromecast can do other casting but I assume those services (Netflix, Hulu, etc) are using more than just DNS queries as well to see what you watch. And if you're casting local media then no DNS goes out at all.
Could be some other motivation, like making it not work in typical corporate environments, where arbitrary external DNS access isn't a given. Perhaps to upsell some other, more expensive device.
(Maybe not this, just an example to note that motivation can be hard to discern)
Google's smart though. They know that rolling out unsavory ideas in small pieces keeps it under the radar.
They didn't, for example, push the organic search results under the fold all at once.
This could be innocuous, or part of some larger agenda.