I would imagine because "two wrongs don't make a right" and because "doing like the others" validates their doing.

Giving up the moral high ground is not a move of a superpower, and aides in destabilising the geopolitical playing field.

Except that the comment at the head of the thread was the one arguing that two wrongs make a right - that if the US does this, then all of a sudden China could start doing the same thing to US companies. The reply is just pointing out that this ignores the fact China already does this, on much vaster scales, and has done since forever. (Given that China is about the closest thing we have to a second superpower, this fits interestingly with your argument.)

Geopolitics is a little more complicated than phrases we tell children.

At the core of it, the kind of protectionism China has engaged in is defecting in a prisoner's dilemma. If you play always cooperate against a defectbot, you're going to lose eventually, even if you start very far ahead.

The correlation of free trade with civil liberties strikes me as missing the forest for the trees. I can’t imagine the average citizen thinks of “free trade vs. protectionism” in moral terms.

Hiding the discussion does not fix the issue.