Why do Uighurs have to file with the police when they go to Sanya to work?

Why do Uighurs have to report to the Public Security Bureau when they go to Sanya to work? Those who return to Sanya from outside the province must report to the local community (village) and unit within 2 hours after their arrival, and timely implement the corresponding health management measures. Sanya is a prefecture-level city in Hainan Province, located at the southernmost tip of Hainan Island, from north latitude1809' 34 "to east longitude1837' 27" to east longitude10856' 30 ". It is 9 1.6 km long from east to west and 5 1 km wide from north to south. It governs four districts. Sanya is an international tourist city with tropical seashore scenery, also known as Oriental Hawaii.