Copying data through a protocol that purports to be integrated with the network frames that copying as a part of that network. If it was acquired through a bridge that does not respect federation then it is dishonestly coopting the legitimacy of the fediverse. Screenshots or copy-pastes won't have the same appearance of integration and will be intuitively understood by the reader as being lifted from another context. This happens all the time and we're very familiar with it. If copying data were all this was about, this solution should be sufficient.
It brings fediverse users into direct contact with non-federated networks in a way that they have not consented to. The ability to post directly back & forth exposes people to the kinds of discussions that we had previously moderated out of our networks. Defederation is an important tool for limiting the access bad actors have to our discussions, and accepting a situation where we can no longer defederate neuters that tool.

This isn't just about "information wants to be free". This is about keeping the door closed to the bigots, and forcing them to come onto our territory if they want to talk to us, so we can kick them out the moment they show their asses.

EDIT:

Spinning up a new instance then federating with you, then bridging the content from there to the defederated instance.

This is exactly part of the problem with a bridge that doesn't rely on federation. With threads, we could just defederate and forget about it. With a bridge like this, we're playing whackamole with every anonymous instance that bluesky spins up, which they can do easily faster than we can detect them.

If this open source system is told to pack its bags and leave, then yes, they can do it more covertly, but if they do that then they're doing shady shit, and that can be exposed as the shady shit that it is. The point of protesting this is saying that we won't allow this kind of entryism to openly exist on the network.