They are, however, being taught things contrary to the Torah. Anything contrary to the Torah is not Judaism. Jews are urged not to attend synagogues affiliated with those movements, but are instead urged to study with the Orthodox. The Rabbis very clearly said that a non-Orthodox Jew is still a Jew. I think the answer is that most people these days are more familiar with Christianity than with Judaism.
Most people tend to think in Christian terms.
To many Christians, if you don't keep Christianity, you are not a Christian. They are unfamiliar with the Jewish concept of inheriting one's race and religion together. Among Jews, the two are intertwined. If you are born Jewish or convert to Judaism, you are always Jewish. If you are a Jewish woman, then your children will always be Jewish.