Not that I'm defending the Mormons, but facts can be helpful. If you read 1 Cor. 15:29—not in the NWT, you can perhaps see why the LDS people feel the way they do. As for Jews objecting, they might consider that the practice of ritualistically doing things for the dead went on in their own tradition too. At 2 Maccabees 12:39-45 Judas Maccabee atones for the sinful dead by sacrifice.
One cannot help but wonder if certain Jews of today are more upset by a group of professed Christians "invading" the Jewish sphere of something many Jews consider holy and thus exclusively their domain.