U.S. Customs and Border Protection accuses Hetian Taida Apparel Co. in China’s Xinjiang province of using forced or prison labor, and last month the agency banned U.S. imports of the company’s garments. It’s a first step, but the U.S. should go much further. China, the world’s largest cotton producer, has built the world’s largest prison system to provide the labor needed to sustain cotton production.

This cotton gulag is primarily based in Xinjiang, home to most of China’s Uighurs and other Muslim ethnic groups. As China...