This seems logical to me. While SpaceX are doing awesome work, they still have a long way to go. When Elon sits $8bn on top of one of his rockets and launches it into space, he can argue NASA should. I don't think, sensibly, SpaceX should want this responsibility at this point.
If it blows, who pays to build another? How long will that take? What science are we missing in the mean time? And probably of most concern to SpaceX, what damage will it do to their rep?