To be fair, it is understandable decision, as it would copy and connect data / posts to other servers automatically. On the other hand most people don't care or understand what this is about and probably never enable it. Still better than nothing in my opinion. Hopefully all new users get a notification and popup to ask if they want to join the Fediverse too. And new user account registration will probably show an option too at registration time.
Meta will probably be pretty cautious and strict about what inbound content is allowed, since they have a global quagmire of laws and regulations to comply with and cannot just open up the firehose without significant legal risk. I'd imagine they'd only accept content from vetted instances that agree to some amount of common policy.
The whole thing is a preemptive move for EU laws requiring "gatekeepers" to allow interoperability with their ecosystem. Facebook is likely to accept any EU registered instance that is also subject to EU laws... and they may not bother with instances from regions that don't require them to do so.