Nothing. Just like the technology that enforces something like "apps cannot record from the camera and train it on AI off-device" or the technology enforcing well-written and clear EULAs.
When an app on my phone asks for contacts access to work, I don't use it. The system works as-intended.
Edit:
> So Apple users can use FB on their phones
According to your logic, this is a net-negative. Is Apple trying to help or hurt their users here?
And when you don't use FaceBook, then Mark Zuckerberg personally feels sad!
No wait, no one cares.
Meanwhile, Apple says "if you try that, you won't get access to all our customers". And FB and Mark Zuckerberg listen to that threat. So Apple users can use FB on their phones, deny access to location tracking and their contacts and feel safe that FB won't ever say "add them back or we'll stop working".
When an app on my phone asks for contacts access to work, I don't use it. The system works as-intended.
Edit:
> So Apple users can use FB on their phones
According to your logic, this is a net-negative. Is Apple trying to help or hurt their users here?