This is a simple duty arbitrage play.
For anyone wondering what he actually did wrong, there are a few key points to note. Aluminum (and all materials, really) is exported and imported and duties are paid on the basis of the HS code and (if there is a trade agreement to be taken advantage of) certificates of origin (COO) along with the usual shipping docs.
China is inclined to tax exports of primary aluminum because aluminum is power intensive to produce. Thus, exporting raw aluminum ingots whose production has been state subsidized is basically akin to exporting (even giving away!) power. However, China relaxed export duties on certain 'finished' aluminum products. This opened up a loophole where Chinese companies could make 'finished aluminum products' which are classified under a different HS code than primary aluminum and ship it out of the country, duty free. In reality, this meant that they sloppily made it into whatever shape wasn't ingots or T bars rather than into any concretely useful shapes.
These 'finished' products then made their way out to places like Mexico and more recently Vietnam. Since they were still carrying Chinese COOs, they couldn't be directly re-exported to the US (or any other country without a FTA with China). They needed to be remelted/recycled into 'raw aluminum' and given a new COO (or for the less scrupulous given a fake COO...), hence the need for the Mexico factory.
So...if cost to ship to Mexico + cost to extrude and remelt < difference in Chinese VS Western manufacturing costs + duties on primary aluminum ... this is a profitable (and legal) trade!
