The reason is that Meta is an extremely harmful company. They’ve enabled the worst kind of people in their pursuit of “engagement”. It’s no exaggeration to say that Facebook enabled a genocide. So if people are (correctly) quick to block instances where fascists congregate, why would Meta be treated any differently just because they have a ton of users? They enable fascists, they provide them with a platform. And now they want to bring that platform to the Fediverse, which has been a place that has traditionally been anti fascist.
And that’s just assuming they’ll be good citizens and won’t do an embrace, extend, extinguish thing, which we all know they will do whenever they feel secure enough in their position to do it. So rather than waiting until Meta is already integrated and it’s harder to do it, the idea behind all this is to prevent the issue from coming up in the first place.
If some random dude comes in and opens a new instance, and then it comes out that this dude willingly associates with white supremacists, is a known creep, and even had a hand in an actual real life genocide, everybody would defederate without a second thought.
But suddenly that dude is Facebook and has a shit ton of money and everybody is just wait and see.