Because you're paying US income taxes while consuming next to no US government services or infrastructure.
> But in scenario (B) you're spending money in Japan
Anyone who lives and works in Japan spends money in Japan. What's great about that? Most of those people also pay taxes.
> basically you're directly injecting US money (your US salary) into the Japanese economy
Japan might say: if this US company doesn't mind someone working from Japan and paying them an American salary, why not a person who already lives there and pays taxes there? That's obviously better than someone new who doesn't pay taxes there.
I think you both are right in a way and what is maybe relevant is the duration.
If someone lives full time permanently in another country working remotely, they probably are already actually a tax resident of that country and would typically pay tax to that country.
What the country doesn't want is someone traveling and in the country for a few months and then taking a local job that could have been taken by a citizen while also not being a tax resident, which is what the work restrictions on visas are intended to prevent.
But if someone is traveling and in the country for a few months, and works remotely while there, it really makes little difference to the country compared to another tourist other than the fact that the visitor now has access to more funds to be spending in their country while there; but visas don't support this well.
Because you're paying US income taxes while consuming next to no US government services or infrastructure.
> But in scenario (B) you're spending money in Japan
Anyone who lives and works in Japan spends money in Japan. What's great about that? Most of those people also pay taxes.
> basically you're directly injecting US money (your US salary) into the Japanese economy
Japan might say: if this US company doesn't mind someone working from Japan and paying them an American salary, why not a person who already lives there and pays taxes there? That's obviously better than someone new who doesn't pay taxes there.