Why? If a nazi baby was born of a nazi Mom and a nazi Dad then baby eventually decided not to be a nazi would he be a sinner?
Its extreme yes. But I could try to be more subtle and work to describe to you my apparently odd point of view but I am more interested in your point.
Why should jews practice Judaism as opposed to say mormonism or atheism?
Why do you feel all jews should practice Judaism?
First of all, Nazism is a political doctrine. It is neither a religion nor a culture. The analogy is simply ineffective.
Judaism is a socioreligious ethnicity. It is a religion and an ethnic culture, both, inextricably intertwined. For Jews to cease becoming Jews is to destroy the culture and religion. Jews should be Jewish, among other reasons, for similar reasons that Navajos should be Navajo, and speak Dinee, and observe the Way; or that Maori should speak Maori and follow the old ways; or that indigenous Australians should speak their ancient languages, and follow their old ways. The inheritors of ancient, beautiful, endangered cultures should not give up on their inheritance and assimilate into the bland pablum of Western Society, but should preserve and pass on their ancient ways, their texts and music, their languages, the unique gifts that they bring to the world.
Second of all, I believe that being Jewish is what God wishes of the Jews. This is what we are here for. It is our part in the covenant, a covenant which we, as an entire society, committed to forever; and individuals do not have the right to unilaterally withdraw from that covenant. Whatever our individual purposes are in the world, we were born as Jews, to fulfill those purposes within the context of being Jewish. To be something else may be just what God wants from other people. But we are Jews: this is what is ours, to do and to be.