HN is weird. Articles, stories, and conversations show up here before getting to content aggregators or social media, and it's ahead of the curve by hours. Focusing a/b tests here gives you a strategic edge in controlling a narrative further downstream, and can give you a sample of what arguments can close down a discussion, or exactly how to script your sock puppets so that people feel their opinion is represented within a discussion.
I'm pretty sure this happens because people in ycombinator circles use HN as a high quality content pipeline, and because of dang's impeccable curation, front page HN content gets greenlit for immediate display. If you see something on HN, it'll almost inevitably show up on Slashdot, Reddit, Facebook, and a slew of other smaller sites and forums, often unchanged from its presentation here.
HN also has the bonus that many silicon valley movers and shakers frequent the site and join the discussions. China's troll generals would be foolish not to capitalize on that.
As far as immigrants defending ccp that's usually because they have family back in China and would like to see them again some day. Their family's social standing could take a hit if they are insufficiently "patriotic," even in the US. China's got social influence inside the US, and extensive networks of casual spies that report problematic behaviors, ranging from local cultural clubs and groups to college and campus outreach organizations. It's very (darkly) sophisticated.