There is no shortage of paradoxes both big and small in the world of Mormonism. One of the big ones is the supreme value placed on the concept of moral agency.
President David O. McKay is quoted as saying that "next to the bestowal of life itself, the right to direct that life is God’s greatest gift to man." More recently, President Monson taught: "When we left our premortal existence and entered mortality, we brought with us the gift of agency. Our goal is to obtain celestial glory, and the choices we make will, in large part, determine whether or not we reach our goal." Elder Christofferson similarly advised that "In matters both temporal and spiritual, the opportunity to assume personal responsibility is a God-given gift without which we cannot realize our full potential as daughters and sons of God."
Every member of the church has heard many times over about how inestimable the gift of free agency is. We all know it's the method by which we can take advantage of the atonement and find our way back to God. We all know it's a crucial concept in our efforts to reach exaltation. And exaltation is a critical piece of why the church's narrative concerning the freedom to choose doesn't add up. Because exaltation is the ultimate goal of our lives. And exaltation means becoming like God and existing as perfected beings.
So how, exactly, do we become exalted? How do we become perfected?
We know, thanks to Alma, that the same moral frame of mind we possess when we die continues with us in the postmortal world. He uses this as a way to tell us not to procrastinate the day of our repentance, but it should also mean that we will be morally imperfect—even if we're well-meaning and fully repented—when we arrive in the Spirit World after death. Since Mormon theology is full of references to becoming gods or becoming like God, it stands to reason that in our exalted states we will exist in the same moral framework that God currently does. This means that we will be perfect and unable to make immoral choices. The problem alluded to by Alma is that we will still have the capacity to choose immorality and the tendency to desire the immoral choice from time to time even after we're dead.
So at some point between the Spirit World and the Resurrection and Judgment and Exaltation, the desire to sin and the ability to sin will both be removed from us. Without intervention from God, we could become exalted beings who can make evil choices—and nobody needs a fledgling god like that. We also know that intervention from God is required because we can still do evil without a devil to tempt us—otherwise there could have been no war in Heaven because all of us (Lucifer included) would have been incapable of desiring and choosing evil without a personification of that evil to influence us. This is why God must remove the tendency to sin and the ability to sin from us after we die and before we're exalted to become like him. That way, when we become gods, we will not be able to do evil or we will not be able to even want evil.
In case it's not already clear where I'm heading with this, I'm demonstrating how a central idea of the Plan of Salvation is a lie or a paradox or maybe just a bait-and-switch. Agency is taught as such a crucial element of our existence and a wondrous gift from God.
But here's the paradox: the ultimate goal of exercising our agency responsibly is to have it permanently stripped from us.