Saying that the bridge is nothing more than another ActivityPub instance is very disingenuous.

While it may be built upon the ActivityPub protocol, but its main purpose is to act as a bridge to non-federated platforms, which is unique to that instance. When signing up for a fediverse instance, it should be known to the user that their data will be shared within the fediverse network. But, no permission is given to share on any platforms outside the fediverse network, using non-ActivityPub protocols.

So, no, opt-in should not be necessary for all instances, but in the case of the bridge, it is, because it’s enabling a feature that users haven’t explicitly agreed too and isn’t a core part of the ActivityPub protocol. And since the bridge is being made open-source, should users also be expected to track down any other instances that pick up and use it and manually block and opt-out of those?