The Mormons are not "claiming" any new members. They are merely ensuring that these people are not left out of Salvation because of a technicality.
As I said before, I'm not a Mormon and I do not believe in Mormon doctrine.
However, I presume that Mormons do believe.
They believe that by doing these post-mortem baptisms, they are allowing these people to enter Heaven.
The Mormons are doing this to, in their belief, save an immortal soul for all Eternity. You are asking them to, in their belief, not to save these souls because it is "tacky" and because it is "insulting and obnoxious" to the easily offended.
Let's see.......Which one to pick?........Save an immortal soul for all Eternity.........Not be thought of as "tacky" by some.
What a choice!
If some Mormon cares enough about my non-Mormon soul and about the souls of my dead non-Mormon ancestors to take the time and trouble to, in their belief, make it possible for us to be saved for all Eternity, I find that quite admirable.
If a Jewish Rabbi, a Protestant Minister, a Muslim cleric or a Hindu Priest were to take the time and trouble to do whatever it is that they do to, in their belief, help me or my family or my ancestors in the hereafter, I would find that quite admirable too.
That is God's province NOT the Mormons. God gave us this time on earth to do His will as each of us discern it. That is the ONLY chance we have. If Hitler is baptized by Salt Lake City, is he now plucking a harp next to Gabriel?