Just having a passport for a particular country is not the single deciding factor to determine entry. There are already other requirements in many countries tha

Just having a passport for a particular country is not the single deciding factor to determine entry. There are already other requirements in many countries that have nothing to do with citizenship. (Think Yellow fever vaccincation in some Asian countries.) There is absolutely nothing that would prevent Japan or the EU to impose additional differentiating factors to US citizens is deciding entry to their country. “For safety reason, we require all visitors to have a valid driving license. We only recognize licenses from those states that have less than x driving related deaths per 100,000.” Absurd from a policy point of view? Of course. Impossible from a legal point of view? Absolutely not.