Part of the prophetic sealing is not only the pronouncement of blessings, but the pronouncement of the cursings. The wo-statements come as a formulaic part of the conclusion to his writing. He must seal the plates, and the wo statements are an important part of that seal. The plates become a blessing or a curse, depending upon how we approach them, and the witness that we receive through faith.
[wo unto them who shall do these things away and die, for they die in their sins]: Those who deny the gifts of God are those who not only do not have faith, but they are those who have not accepted Christ. Therefore, the atonement from spiritual death will not apply. They may be resurrected, but if they die in their sins, the implication is that they have not accepted the sin-offering of the Atoning Messiah. They therefore remain as though there had been no atonement.