Thereby ruining the very thing they want to control and profit from.
That's the thing that the "They'll just send in the army, LOL" folks don't seem to get: bombed out cities and a population under armed guard aren't very economically productive and, on top of that, you need to station military units there to keep a lid on things which is also costly.
As this vox article states correctly, Hong Kong used to make up 1/5th of their entire GDP. But with expansion and growth in cities across the country, it is now but a minor component of their economy. In the grand scheme of things it isn't a large enough chunk of their economy that they couldn't afford to lose it. Especially not if the balancing act is, bomb this one city and regain political stability vs massive instability for the greater entity.
[0]https://www.vox.com/2014/9/28/6857567/hong-kong-used-to-be-1...
For Beijing, the destruction of some parts of one urban may be exactly the price they're willing to pay to maintain their grip.