Even though G-d confronts Sarah, she isn’t punished for laughing. Maybe her laughter made sure she got her baby. Maybe her laugh was like a dare, and the challenge made G-d really want to make good on the promise.
The origin of the idea that Abraham and Sarah were so welcoming of guests actually comes from this week’s Torah portion, Vayera. The portion begins, “And the Eternal appeared to him [Abraham] in the plains of Mamre; and he sat in the tent door in the heat of the day. And he lifted up his eyes and looked and lo, three men stood by him; and when he saw them, he ran to meet them from the tent door, and bowed himself toward the ground.”