Linux is well established - the licensing terms are unlikely to change and the community understands (independant of the legal agreement, which they mostly don'

Linux is well established - the licensing terms are unlikely to change and the community understands (independant of the legal agreement, which they mostly don't) that once they contribute code it is out of their hands. The business of Diaspora is just as green as the code of Diaspora, and requiring copyright transfer is a very sensible way of anticipating the inevitable pivots.