The concern is that the fediverse as it exists now may not be big enough to survive corporations entering it. If they start creating their own modified versions of that open protocol (which they will) it will eventually mean they start breaking compatibilty with the rest of the fediverse. That means that the rest of the fediverse will then need to move over to their protocol which they have control over or just accept that the free and open fediverse can no longer connect to the corporate fediverse. Right now the free and open fediverse is small and may not have the staying power to survive that disconnect once people get used to being connected to the corporate side of fediverse. Once that disconnect happens the average joe is going to go with the side that has more content which is going to be the corporate fediverse if they are alowed to join before the free and open fediverse has truely gotten off the ground.

Meta has also never been anything other than actively malicious when it comes to online communities. Giving them any power at all in the still infant fediverse brings more risk than any possible reward. It's like letting a hyena into a daycare and saying it's fine because the hyena isn't hungry right now. Eventually it will be and why would you even let it in in the first place. Eventually meta will want more control. Letting them in now and allowing them to cement their place in the fediverse will just make it that much easier to extend and extinguish when that time comes.