They have the right to make dangerous crap wallboard for the Chinese people? Do the Chinese people and officials know about that right?
Why would they make that disgusting garbage for the locals while making much better stuff for export? You’d need seperate factories or at least seperate lines. The workers would sure know what was being done.
Why would anyone believe that, or trust any wallboard from China?
Simple, China is a communist country and communist governments don't give a damn about the population of their countries. Exported products, however, need to meet standards imposed by the importing country. It's also why the best BMWs are only available in Germany, US CAFE and EPA restrictions reduce the power in exchange for MPG.
There's no evidence that this stuff is dangerous, only that it discolors copper plumbing and electrical circuits, and tort lawyers are gearing up to sue anyone connected with it - from domestic firms, from which they can collect, to foreign manufacturers, which are immune.
They make this stuff with the full knowledge of Chinese officials, just as the same Chinese officialdom knows that Chinese cars made for the local market will pass Chinese crash tests, but not American crash tests. This is why you don't see Chinese cars on American roads, because car imports are scrutinized, whereas wallboard imports have not been. But the Chinese have no problem buying an inferior $5,000 Chinese deathtrap with an 0.8L engine because it's safer than the alternative - a moped.
As to production lines, Chinese industry is not composed of a single company. There are large numbers of small, privately-owned companies in every industry. Some specialize in Chinese grade drywall. Others may make only export grade drywall. In a country with 100 car companies (think the US car industry before consolidation into GM, Chrysler, AMC and Ford), I expect there are thousands of drywall companies at minimum, given that it's not exactly high-tech.
And if you think it's impossible for a single company to make both high and low quality goods on a single production line, let me point out that drywall is of a uniform size and shape. It's the equivalent of saying that a sawmill cannot process both oak and pine timber. (I have it on good authority that Procter and Gamble feeds different grades of product through the same production line. They run on different shifts and just swap containers and ingredients).