They don't, though. In China, they call it the "June 4th incident," and acknowledge that hundreds died around Beijing that day in conflicts between rioters firebombing the PLA and the PLA's response. What the CPC censors is the now disproven western reporting of thousands dying, and the idea of the square itself having a massacre. Even Wikipedia backs that the square was dispersed bloodlessly. This gets twisted by western media into "censoring the massacre," when in reality it's censoring western misinformation meant to delegitimize China's government.