[CharlieDigital]

Seems like the restriction would only make sense in the context of working for a Japanese company. Similar to US visas for visitors where the traveler cannot visit the US for work purposes with a US-based employer.

Otherwise, opening a work laptop and answering some work emails for your foreign employer would be risky.

[VincentEvans]

Precisely this - countries protecting their job markets against foreigners working illegally (not paying taxes, undercutting wages, etc).

This has nothing to do with you working for your own foreign employer while on vacation.