The anticompetitive part refers to social VR. Today we have multiple social VR apps (Rec Room, Altspace, Bigscreen and more). While Facebook aims to be a big player in social VR they're not there yet; their Facebook Horizons app is still in closed beta.
Requiring a Facebook login to use the Quest puts the competition at an immediate disadvantage, as they'll have to use their own login or submit to Facebook login as well, often at the cost of cross-platform compatibility. Onboarding friction will always be less with the app where you already have an account.
But seriously what a dark pattern and just like Little Shop of Horrors feeding the beast really doesn't benefit you. This is exactly why I never purchased an Oculus: because the price of entry for playing a VR game is too high.