Since it is within their rights to decide the definitions of "Jew", it is within their rights to say that a person who declares loyalty to another religion is no longer a part of the Jewish religion OR the Jewish nation.
Especially considering that since many Christians believe that through Jesus is the only way to attain eternal life, Jews might consider such coverts as a 5th column within their nation, diluting and diverting their national interests and energies. And given the attempts to destroy the Jews through war, violence and conversion over time, I can imagine that most Jews would be sensitive to the attempts by Christians to destroy their religion one person at a time.
If so, isn't this a mistake on their part. Haven't atheists also had great success in leading Jews away from their religion. And haven't atheists had pretty terrible relationships with Jews, too. I am thinking here of persecution of religious Jews by the Communists and of Marx's extremely negative views of Jews.