From what I have read, I think it's all of the above.
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a space is wanted free from corps, ads, data perversion
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people are fearful that 30 million people joining threads has automatically made it the largest instance. Once it integrates with ActivityPub and can federate, it will dominate the space and produce the majority of the content. People are fearful then meta will retract it/ defederate and take the majority of content and content production with it (EEE). This would effectively kill the fediverse.
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many believe meta will not act in good faith and is doing this to appease European courts and laws
Because of all of this people likely believe keeping threads quarantined right off the bat is the best solution to mitigate the amount of damage they can do to what's already been established.
Edit: I am adding to this post as I just stumbled across a post from the host of the lemm.ee instance (which I am a big fan of). He has also listed some great cons of Facebook stepping into the fediverse:
-there is nothing stopping facebook from sending out ads as posts/comments with artificially inflated scores which would ensure they end up on the front page of "all" for federated servers
-threads already has more users than all of Lemmy's instances... therefore, they can completely control what the front page looks like by dictating what their users see and vote on
-moderation does not seem like a priority for threads which would increase workload for smaller instances
-REVENUE FOCUSED
I paraphrased a lot of this but as this is getting some traction I wanted to provide additional visibility to the cons of federating with the Facebook.