But forced involuntary conversions of Jews were de rigeur throughout history, usually accompanied by at least the threat of violence, and this all rather harkens back to that, albeit without the violence.

In addition, genealogy is very important to both Jews and Mormons. Some of the largest sets genealogical records in the world belong to the Mormon church. If they start polluting those records with religious falsehoods about Jews, real-world things like the Israeli Law of Return can be affected.

I don't think anyone is scared that they are no longer going to be Jews in reality because of that sort of totally involuntary "conversion", but, rather, that others will be misled to think that they aren't Jewish, and also, it's just incredibly presumptuous to convert someone when the only possible explanation is that they think that the people being converted were too dumb to join the Mormon faith. If they had wanted to do that, then they would have done it themselves. The whole reason the posthumous Mormon baptisms exist is so that people who were unable to join the LDS church, but were likely to have wanted to could do so, like infants and the ancestors of current Mormons.

You also seem to assume that a rite in one branch of Christianity should be equivalent to rites in other branches, and that's not true. Most (if not all) people who convert between branches of Christianity will rededicate themselves in the new branch. I'll give you that a Methodist converting to Presbyterianism probably isn't likely to, but certainly someone crossing between the Mormon/Catholic/Protestant barriers would.