Earlier quoted context omitted.
I would guess that those are, more often than not, not paid shills but rather just people who like taking devil’s-advocate sides in arguments “for fun”, i.e. just for the challenge of seeing if they can construct a valid rebuttal to a comment. (I say this because I believe I’ve done this myself more than once, and I know my personality-type isn’t uncommon on HN.)
A simpler explanation is that they are individuals defending against what they perceive as a personal attack via 'tribal' affiliations.
I've commented here defending China on some issue a few times in the past. I have no ties with the country (except being on a business trip in Shenzhen for two months), nobody paid or even offered to pay me to do it. It's just I hate bullshit, especially when it becomes popular opinion. Just because China has serious issues doesn't mean it's the Devil and everything it does is evil. It's a country, not unlike Russia or the US.
(Particular things I've been addressing in the past are a) treating a billion+ population as if it was a small country, and b) blaming China for all cheap mass produced crap and its environmental impact, conveniently forgetting that they aren't doing it for fun, but to fulfill orders from Western companies.)