First, most J4J aren’t Jews, although a minority are. They are Jewish in the same way as George Soros or Noam Chomsky. If they return to the practice of Judaism, they don’t need to convert, and their children will be Jewish if their mothers are. They are always welcome back.
It is true that a Jew who openly espouses another religion loses certain privileges connected with being a Jew and so may not be treated as one in certain circumstances, but the essential Jewishness is unaffected.
Most “Jews” are atheists, so by your reckoning they are not Jews either. I find it odd that men think they know better than God Who decided at our birth who was Jewish and who was gentile.