I answered a question very similar to this recently (link is below). The title is justified by the line that Joe Keller has late in the play: “I think to him they were all my sons. And I guess they were, I guess they were.” What Joe is referring to is the fact that he should not have only been thinking of his biological sons when making the decision about the faulty parts. He should have been thinking of any airman as his son - that they are all his sons. His moral responsibility extends beyond his immediate family.