Once upon a time, the US agreed to these rates, because at the time they were reasonable. The idea was that developing countries would be held back if they had to pay the full cost of international mail. To help them, the relatively low volume of mail from developing countries would be subsidised a bit by the country it was mailed to.

But China isn't a developing country anymore; it's the #2 economy of the world. And the volume isn't small either. The rule should not apply to them, but it's hard to change.