According to Rodney Turner, a major doctrine of the Book of Mormon is that as a result of Adam's transgression, "all mankind became a lost and fallen people" (Alma 12:22). Exactly why one man and woman's transgression led to everyone's transgression is not explicitly revealed in scripture. However, God is just; we are no more accountable to him for someone else's acts in one estate than we are in another. Possibly, the Fall affected all people because all people--exercising their agency--became willing co-partners in Adam's foreordained fall. That is, humanity covenanted to accept the Fall's temporary negative consequences (spiritual and physical death) so as to obtain its ultimate blessings (immortality and eternal life). [Rodney Turner, "The Infinite Atonement of God," in Studies in Scripture: Book of Mormon, Part 2, p. 38]
Note* Additionally, as co-creator of the world, would not Adam (Michael) along with Christ (Jehovah) have the right to act in behalf of that world? [Alan C. Miner, Personal notes]