The allegation isn't that they denied or never apologized for "Japan's actions in China/Korea". The allegation is that they don't acknowledge the Rape of Nanjing (or the crimes of Unit 731).
Imagine Germany had at numerous times apologized for WW2 and "the actions committed" without ever explicitly acknowledging the Holocaust, the mass murders in Poland and other annexed territories and so on.
Without explicitly acknowledging what Japan had done in Nanjing (or what Unit 731 had done as "experiments"), an apology means nothing. It's the difference between apologizing for a war and for the war crimes committed during that war. It implies that everything that happened can simply be paraphrased as "actions during wartime", which understates the extent of the heinousness of the crimes.
Imagine Germany had at numerous times apologized for WW2 and "the actions committed" without ever explicitly acknowledging the Holocaust, the mass murders in Poland and other annexed territories and so on.
Without explicitly acknowledging what Japan had done in Nanjing (or what Unit 731 had done as "experiments"), an apology means nothing. It's the difference between apologizing for a war and for the war crimes committed during that war. It implies that everything that happened can simply be paraphrased as "actions during wartime", which understates the extent of the heinousness of the crimes.