I appreciate that you're trying to maintain HN as a place for civil intellectual discourse, but I'd argue that people in these threads constantly implying China to be beyond criticism because "well the US does bad things too" is the actual "tired trope" here, not the people rightly calling out that fallacious logic and labeling it as what it is.

They both are. There's no law of conservation of tired tropes.

Fair enough, but at the very least both should then be called out as such, no?

Sure, but we can't come close to moderating everything, or even seeing everything. I've certainly replied to many such comments on HN. If you see a particularly bad one that didn't get scolded, the likeliest explanation is that we didn't see it.