Go back to countries in the same religious/ethnic tree meddling in the Middle East in the early and mid-20th-century and you can link back a whole host more problems to white Christians!
But the southern evangelical side of my family gets pretty upset whenever I point that out...
Feigning to not know, is knowing.
The truth unfortunately just doesn't matter much these days.
Folks read stuff online by people and just take it as fact without bothering to ask the simple questions that you have.
You need Japanese blood (visibly), culture (language, manner, custom), and nationality to be "fully" accepted. This isn't new and has more to do with Japanese people themselves than with any legal restriction.
What confuses me is that Japan is not a very immigrant-heavy country. There's no chance of someone who doesn't represent the interests of native Japanese people being elected any time soon, or ever. So why can't they pass laws codifying the behaviors they want to encourage and ones they will treat as non-assimilation? Why aren't they doing something to address misbehavior directly, making newcomers to the country prove their long-term commitment to their new social and cultural environment, or punishing infractions more harshly so bad actors can't stay in the country? Why are the only two options available to them 'do nothing' or 'blanket punish all foreigners because all foreigners are trouble'?
Japan is a shame based culture. A lot of social norms are encoded as part of being Japanese and not strictly enforced. Japan has less lawyers per capita compared to US for example.
Take a look at a lot of the influencers that are causing trouble in Japan. In the US we probably bludgeon (or shoot) those people on the spot and lawyers talk later, but Japan doesn’t do that, not even their police. Majority of Japanese just sit there silently but deep inside filled with disgust.
Besides, once these laws are implemented, soon it will be obvious which groups are “targeted” and it doesn’t look good in the eyes of the international world.
Besides, the implication you made is that a lot of the actions people don't like are already illegal, just not tightly enforced. Especially if you say Americans would shoot the perpetrators on the spot.
Even if they wanted to protect their status quo, they could only apply stricter requirements on residents who haven't received their citizenship yet. Law-abiding immigrants in many countries are used to having to clear higher bars than locals. But at least in this case this would be equitable, only punishing the people who actually do something wrong instead of everyone.
The US could avoid a lot of inefficiences if they adopted the same.