Maybe they single out China because they're relaying their personal experience with a seller who happens to be from China.
Consider, the story is just as powerful with "I ordered from a vendor who had a ton of negative reviews for not delivering products." Even better, that instructs to start paying attention to reviews, and not origin of vendor. :)
If the original comment had mentioned how the purchase was from a black vendor, or a gay vendor, people would be up in arms about what that unjustly implies, and how without any sort of data to back that it's just stereotyping (at best). Do it for China, where people already have a strong preconception of the types of vendors present, and all that rational separation of anecdote and data and needing to justify your statements goes right out the window.
This isn't to say you can't call out vendors from China if you suspect they are a problem in higher proportions than other vendors. The original comment could have included "I suspect this may be a problem exhibited more often from vendors from China, but don't have data to back that up" or more strong statements with some data, but did not. Instead we have a statement that only adds to the comment when combined with your own preconceptions, since it adds nothing to the circumstances of the described situation without your own preconceptions.
> rational separation of anecdote and data and needing to justify your statements
No statement was made though. An anecdote simply provided additional data, and you objected on the basis of what people might assume.
> but don't have data to back that up
You might also assume that from no data being provided, and indeed, no claim being made.
> we have a statement that only adds to the comment when combined with your own preconceptions
or allows another thread on the topic to emerge, in this case if it's relevant that China was involved, causing someone with knowledge on the topic to comment.
If you manufacture in China you find huge legal and social issues with maintaining quality. It's still possible just far from the default.
I've read, without adequate if any sourcing, that this is a pattern that's at least centuries old, specifically, building things to the required specs, then backing off on quality until the buyer complains....
If so, it's interesting it survived the harshest decades of Communism and the attempts to do away with the old ways, then again, it could well be aligned with the incentives that were force back then. Although I'd hate to be issued a weapon or ammo that was from a factory running along those quality lines. Although back when we were allowed to buy ammo from the PRC, that sort of thing was not apparent from what I remember, but I'm not sure I would have noticed since I was never in the market for those particularly types.