Apple was going to make the peer-to-peer video call technology they were using for FaceTime public. Then they got sued by VirnetX over a patent related to that technology, so they had to rework the service to run through a central server. With that change, there was no longer anything worth open-sourcing.
It's open and federated protocol, not open source, that everybody else wants from them.
Which is a legitimate complaint especially since Apple had said they would be opening it up (or was that FaceTime...I'm not sure).
But that's a wholly different complaint from the fact that Apple, like nearly every other messaging service, does not allow you to export groups to other services.