Actually, it has more to do with crops purposing than anything else. In the US, most of our wheat crop goes into bread, for which the entire grain is used. No leftover wheat gluten. In China, a sizeable portion of their wheat production goes into starch production, for all the food purposes you need starch for. What's left over is wheat gluten, which they sell off for pet food and other products.

In the US, most of our starch production comes from corn, which, unsurprisingly, doesn't have wheat gluten. Mass quantities of corn grow well in the US, mass quantities of wheat grow well in China. It's in everyone's interests for China to sell their surplus wheat gluten, and it's in our interests not to overproduce our wheat just to get more of the "trash product" gluten.