I think the fear is that this turns into an "embrace, extend, extinguish". https://en.m.wikipedia.org/wiki/Embrace,_extend,_and_extinguish
I don't know if the fear is well rooted, but I can definitely understand how Facebook is perceived as not having established a history of trust.
They are a private company, which have placed profits above the best interests of its users.
Edit: I think you can draw a parallel with another scenario: an open and free market requires regulation. There should be rules and boundaries, such that a true free and open market exists. Similarly, there's an argument to be made than we should restrict the fediverse for it to keep existing in the way we want it to.