The fediverse is too new and niche to say that with certainty.
The legality is likely untested and certainly not enforced by pubspec yet.
I don't know enough to speak to the technicalities with certainty, but my surface level understanding is that that is exactly how it works, and it is one of the known flaws of the fediverse as it currently exists.
You might be making a statement, but server B is just a node and, frankly, doesn't care. If you federate with them, you federate with everyone they federate with as well.