Because commenters outside Japan may end up buying products containing chips made in Japan. If it was built in let's say France people would be thinking less about potential invasions. Just as "obviously Japan is going to want to develop lucrative manufacturing within Japan," obviously people outside of Japan are going to want manufacturing that is not liable to be shut down or taken over in some way. Not that I think Japan and China will actually go to war any time soon myself.
>geopolitical narrative fed to them by the US state department
Just this week Japan and China have been getting into a fight over the current PM's comments over Taiwan. China has canceled some flights to Japan and complained to the UN, announcing it will defend itself from Japan.[0][1] I'm not sure what point you are trying to make here. Are you saying major disputes between China and Japan don't exist and are invented by the US state department? Or that thinking about it in this context is the result of the commenters being fed by the US state department?
[0] https://www.scmp.com/news/china/article/3333992/china-blasts...
[1] https://www.reuters.com/world/china/china-takes-spat-with-ja...
The PRC and Japan is not a remotely comparable situation to the PRC and Taiwan.
The most the PRC could do is potentially sabotage production in Hokkaido, but if they can sabotage production in Hokkaido, they can sabotage it in Arizona.
Imagine HN was Japanese and everyone was talking about how the US was threatening to invade Greenland on a topic about a new plant in Montana.
Fun fact, there's more precedent to Russia successfully invading Paris than Japan. Although to be fair, they had help.
https://en.wikipedia.org/wiki/War_of_the_Sixth_Coalition