BEIJING — Kayrat Samarkand says his only “crime” was being a Muslim who had visited neighboring Kazakhstan. On that basis alone, he was detained by police, aggressively interrogated for three days, then dispatched in November to a “reeducation camp” in China’s western province of Xinjiang for three months. There, he faced seemingly endless brainwashing and humiliation, he said in an interview, and was forced to study communist propaganda for hours every day and chant slogans giving thanks and wishing long life to President Xi Jinping. “Those who disobeyed the rules, refused to be on duty, engaged in fights or were...