Nothing stopping them, except, you know, the law… They can certainly display content that was not marked for public display. They will then proceed to get sued out of existence… If they do this automatically I’ll just privately post a music file with copyright protected music. Which is perfectly fine to do if it is indeed hidden from everyone. If they then publicly post it that’s on them and now I get to see the Music Industry fight the Zuck :D

The largest instances have either already announced their intent to block Facebook or stated that they are monitoring the situation and will react quickly and decisively should anything untoward happen. There is no Fediverse without federated third parties. All Facebook could show in that case was… Literally their own walled garden. How is that different to them not even implenting activitypub in the first place? It isn’t. Their only power is to ask if they can participate. Literally no one is going to waste a minute on any efforts of theirs that could even remotely be perceived as taking control.