I would posit that they are in fact harming Jews by doing this. It dilutes the Jewishness of the Jewish holidays. How long until Hanukkah is no longer considered to be specifically Jewish? How many centuries might pass until Jews are banned from celebrating the “Christian” holiday of Hanukkah, or for celebrating Hanukkah incorrectly by not lighting the shammash in the name of Jesus?

One could also argue that incorporating Jewish holidays like Hanukkah, Passover, or Shabbat makes Jews more susceptible to Christian missionaries.

It’s happened before. The pagan festivals of Saturnalia and Yule were co-opted by the Church to create Christmas. Those holidays were lost to the pagans, and before long the pagans themselves were no more.