and it isn't
The work can only be done for ancestors WAY back - and even then, is only as an offering, as it were.
As Mormons, (Latter Day Saints) believe in Jesus and that everyone continues to live after 'death' = they believe that those ancestors who have passed over - still retain God's greatest gift, Free Will, and they can then accept or reject the offering...
That's very complicated; you wonder why they bother.