Not sure the full motivation behind this but this is very annoying for a parent with kids. Especially if you've lost your remote, or want to quickly find something and cast it to your Chromecast/Apple TV. I'd prefer video streaming apps be required to support Casting/AirPlay.
This feature was one of the main reasons how Netflix movies/shows could be captured/uploaded to piracy sites in full quality as soon as they were released. People recorded the video output and uploaded them. This is to remove that path. Kinda obvious if you ask.