Having multiple branded builds to choose from would be a terrible thing and would easily allow fake apps to gain traction.
> ... and if you rebrand he forbids you from using the official Open Whisper servers.
This seems pretty fair to me. Not only could you abuse their resources, it would greatly hinder their ability to make changes and respond to protocol-level security threats. They aren't in the API business, controlling their ecosystem allows them to make forward progress without concern for 3rd parties that they have no control over. And still there is the issue of 3rd parties abusing their server resources.