Then your complaint is about Apple locking iMessage to Apple devices.
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.
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.
Except for the vendor lock-in, which makes this a problem to begin with. Other services work on a majority of devices.