I think one of my personal issues with allowing Meta to access these private APIs is that they may not take "don't allow" for an answer.
I have WhatsApp to talk to some family and I recently disabled allowing all contacts in iOS 18. WhatsApp now has a persistent notification at the top of messages to "Allow All Contacts".
If Apple allows users to choose whether or not to give Meta access, and users choose "no", Meta can lock them out of the service entirely (e.g., "you can't use this Meta Quest headset without allowing access to your messages").
That being said, Apple is definitely fighting for its own interests here as well. It would obviously benefit them to sell their own watches, headsets, earbuds, etc.