The police in America have dedicated teams tasked with taking on protestors and have effectively infinite resources. Chinese police have the same training and resources, but are less constrained by the legal system and have more control over the media.

The typical American probably sides with the police over protesters in most situations.

The only protesters that can match the police are those with inside support. It's how certain people can demonstrate while carrying rifles without issues, while other unarmed peaceful protesters can be corralled, brutalized, and arrested.

It is no coincidence that private gun-ownership is effectively illegal in China, while in the US it is a secured right in the constitution.

An American can sue, use the media, and vote. None of these remedies are available to Hong Kong. There is a rather large difference between protesting in China 2019 versus USA 2019.

We aren't talking about America or Americans. Not that cops in USA restrain themselves...

Hong Kong is more or less an island, a small one. China can fill the "1,104-square-kilometre (426 sq mi) territory" with soldiers and police in a heartbeat. Nothing you can do, cops already know the 1000+ leaders of the movement and they will be arrested or killed in the first hours. Etc etc