And why is that a problem, exactly? In the end aluminum is fungible for aluminum, and the US is getting aluminum from Mexico under NAFTA. Trying to enforce a tariff on element production from a subset of external countries seems sisyphean.
But there are essentially no tariffs any more. Not sure how NAFTA changed that. But assuming its true: aluminum is fungible as mentioned. Providing more to Mexico means they can export more of their own to the US.
If China is greatly subsidizing the aluminum production then then how will US producers compete?
It's in the US's interests that they don't allow another country to manipulate the market to destroy the US production capacity. Once that capacity is online the foreign country can then raise prices.
It's not easy or cheap to bring production capacity online.
It could be well within the Chinese government agenda to have broader control over global productive capacity. Impossible to prove, I know.
Generally I'm pro-free trade, but the issue gets quite messy when you consider government subsidies. As a Canadian, this discussion comes up frequently along the lines of the subsidized agriculture sector.
Worse still, China could actually start enforcing their restrictions on exporting raw aluminium and drive non-Chinese manufacturing industry out of business by cutting off their supply of feedstock, as they also tried to do with rare earth metals.
