Both SK and Japan has complete agency about their foreign policy. They can easily impose a govt decree to kick out US forces in heart bit. Check how a weakling like Niger did it.
SK and Japan just have a worldview of western worship where everything US and western is seen as superior and they feel happy to be pawn of the US. You can talk to any South Korean about how they view US compared to China and their distorted view is quite apparent.
SK also feel China to be huge threat cause they think inferior Chinese are rising up and they don't like that one bit. They want US to be in SK not for North Korea, but for China.
Its not a surprising worldview when even ethnic Chinese Hong Kongers and Taiwanese have that kind of worldview. Its just a matter of how wealthy and powerful China can be. If China becomes as wealthy as western europe and becomes superior to SK and Japan in every way, you will see an automatic transformation in worldview and subsequent change in foreign policy.
I would add that in the event of a Taiwan blockade, it is not in China's interests to be cruel or vindictive.
1. As per Gaza's deliberate mass starvation, this is acknowledged to be a war crime.
2. On a practical level, mass starvation in Taiwan doesn't look good in the global media
3. You have all sorts of personal ties between China and Taiwan. For example, as far as I'm aware, Xi Jinping's sister still lives in Taiwan
4. Taiwanese are recognised as equal citizens to anyone in China, albeit with some caveats.
In comparison, suppose China was to flood Taiwan with aid ships carrying food to a hungry Taiwan?
1. It looks better in the global media narrative
2. It helps maintain domestic Chinese support for a war
3. This would likely be a propaganda coup in Taiwan. Remember lots of Taiwanese have a really distorted view of China. China's goal is to win "hearts and minds"
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On the other hand, if Japan were to declare war on China, it would be a different story.
NB. As per McNamara's comments, the US bombing of Japanese cities like Tokyo or Hiroshima back during WW2 was also a war crime, and the personnel in charge of this knew it when they gave the order. However, the lesson here is victor's justice means you can get away with it. And in the aftermath, Japan became a compliant vassal state of the US.