@jcastroarnaud@snarfed.org@snarfed.org it basically is opt in because someone on one side or the other will have to request the content, either one of your users requests a bluesky account or a bluesky user requests your account.
And the assumption is perfectly reasonable given that it's essentially just another instance, as if Bluesky spun up an activitypub endpoint themselves. The opt-out is at least a nice gesture (though unnecessary because blocking a server is an opt out regardless).