Apple doesn't know what every app does, they basically know what frameworks/syscalls get made. You can wrap entitlements around those things without explicitly knowing exactly what an application is actually up to.
Did I feel good about inviting Facebook into my living room? No. I don’t trust that company. But the tradeoff was worth it for me.
Edit: I didn’t just buy one Portal, I bought 4. One would be kind of pointless.
If you want Facebook on your iPhone to call someone then Apple provides a Phone API.
If you want Facebook on your iPhone to stream to your TV then Apple should provide an Apple TV API for Facebook to use.
There is no reason whatsoever for Facebook to have any direct access to anything on your phone whether its your sensors, your microphone, your video, your network.
There is - AirPlay doesn't require local network access since it's an API apps can use. Un/fortunately, there are multiple competing standards like Chromecast (and other, manufacturer-specific casting standards) which require the app to do its own local network discovery to find available devices.