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Netflix Starts Booting Subscribers Off Cheapest Basic Ads-Free Plan
(www.macrumors.com)
This is a most excellent place for technology news and articles.
The ads are added in the app. If you cast, the Chromecast can't add apps (yet) so they'd have to make ad streams instead, and switch between the streams show-ad-show which would take several seconds of loading screen each way and so on. Which is a level of fuckery even they shied away from.
TLDR they can't (easily) show ads during casting.