Basic information missing
This is a law that is sourced back to the Talmud (soon after the Jews were exiled from Israel). It was instituted as a result of persecution by non Jews, and non Jewish authorities. Among the reasons for this Rabbinic law explained in the Commentaries on shulchan Aruch (if I remember correctly the Shabbatai HaKohen), is that once a Jew is in the hands of non Jewish authority (even for a legitimate reason) - they will make new libels against the Jew.
(Other sections could include how the law has a history of shielding Jews from unfair treatment and persecution over the years, including in recent history when Russia and the Soviet Union would use Jewish informants (those who had already given up their religion) to dress like the Jews and spy on the community's religious activities - usually resulting in members being sent to prison - unlikely to come out alive.)