You replied to the message about foreign passports which're mentioned, so don't derail with your "but" or "I suspect" there! If foreigners are asked to "surrender their passport to the local police" there, they are wrongfully requested to do so. Of course everyone has his/her choice BUT when the authority puts one in the situation to break laws ............

I Wrote:
The foreign passports belong to governments that've issued them and so the people that've been issued those documents may have been terrorized by the local authorities there

You Replied:
Interesting argument for foreign residents of Xinjiang. But this fact certainly justifies the
Chinese government holding Chinese citizen passports, since China owns them.

Foreign visitors routinely need a special resident permit to reside in Xinjiang. I suspect
that they are now being advised at the time of applying for the residence permit that
they must surrender their passport to the local police if they choose to reside there.