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.
The AUR is basically just a script that describes best case scenario for building something under Arch. They don’t have any specific quality rules they have to meet.
It’s super easy to make and publish an AUR script compared to a regular distro package (including Arch packages).