What the Mormons do is what the Mormons do, they perform a valuable service by preserving these records and they do it for NADA.
Actually, they're not exactly doing it for nothing. They're providing a service in exchange for your lists of relatives, for whom they will later be baptized in proxy to make posthumous Mormons out of them (if they choose to accept it in Heaven, etc etc etc). Baptizing themselves for your dead relatives is a type of missionary work for the LDS.
If you don't want your great-aunt Martha to be placed on any LDS posthumous mailing lists, don't accept any free services from them.