Leading sponsors of the Beijing Winter Olympics should explain why they remain largely silent about alleged human rights abuses in China with the Games opening there in just under three months, Human Rights Watch said Friday. The rights group said in an online briefing that it had reached out to all but one of the IOC’s so-called TOP sponsors—and leading broadcast rights holder NBC—in lengthy letters almost six months ago. The only reply came from sponsor Allianz, which it wrote only last month. “We stand behind the Olympic Movement and our longstanding support for its ideals will not waver,” Allianz said. The Beijing Games open Feb. 4. The letters asked sponsors to be aware of the rights climate in China, and to scrutinize supply chains and other operations to assure they do not “contribute to human rights violations.” “There are just three months until the Beijing 2022 Winter Olympics, but …