I think the monitoring is secondary. The principle function is intimidation.
It's about as far from Hong Kong to Xinjiang as it is from Seattle to Anchorage. Except Anchorage speaks another language, worships another god, and has another heritage. Kinda like it did 100 years ago. The US did heinous crap to everyone indigenous until the indigenous people were mostly gone; the old colonial powers get to appeal to the moral high ground now because all their crimes against humanity are past the statute of limitations.
China wants a unified "China" to take on the rest of the world. They have neither the time nor the patience for anything but homogeneity. Historically, China has not been about individualism or change; one of the tenets of Confucianism is that technological progress is an affront to the divine perfection of the Now and that spirituality lives in continuity.
I read an interesting book last year.
The mile-high conventional wisdom on the Opium Wars is "Britain fired cannons at the Chinese over their right to sell drugs." The conventional wisdom is bullshit. The reality of the situation is the Chinese allowed everyone to trade in a tiny little corner, under tight supervision, subject to revocation at any time, and no one was allowed to leave. The Chinese were the acknowledged rulers of all the world, they accepted foreign tribute, and they'd occasionally suffer the British because the British bought a bunch of tea. However, a great deal of trade off the books occurred in opium, because the Chinese weren't officially allowed to trade in opium, so they unofficially traded in opium, until the Chinese got a new ruler, who decided opium was the worst thing that ever happened to China and that it was also all the fault of the English, so he confiscated all the opium and blockaded the English.
In effect, the Opium Wars were the Chinese saying "Who run Bartertown?" and the English saying "not fuckin' you lot" and the Chinese launching on a decades-long quest of soul searching and recrimination over having lost their position as the center of the universe. Mao was their first real strike back against the West. Then they shifted to a market economy under Xiaoping. Now they're making a run at being a world power; the Japanese determined during the Taisho period that with the West, your choice is empire or colony; the Chinese were an empire, became a colony, and would like very much to be an empire again.