Ok, actually they did do quite a few things wrong – e.g., most things under Maoism – but they did absolutely nothing wrong specifically 30 years ago.
It’s an artificial outrage drummed up as part of America’s Sinophobia campaign.
There was no such big outrage or sanctions when South Korea killed thousands of protesters in the 1980s, whereas only a few hundreds died in Tiananmen. Nobody in China cares about Tiananmen apart from a small crowd of ageing pro-Western dissidents. So it is obviously played up for ideological reasons – for the cause of Taiwanese svidomism, for Japan to minimize its own crimes against China (as if killing 30 million in a war of aggression is comparable to killing 300 in putting down sedition), and to aid America’s attempts to isolate and contain China now that it is becoming a discernible threat to its global hegemony.