Because:

Due to an amendment in December 2018 of the Unfair Competition Prevention Act in Japan, certain gaming-related activities and services have now been declared illegal. This includes:

• Distribution of tools and programs for modifying game saves
• Selling product keys and serials online without the software maker's permission
• Game save and console modding services
  As such, sales of products such as Pro Action Replay and Cybergadget's "Save editor" have been discontinued.

https://www.reddit.com/r/emulation/comments/abk551/save_game_editors_and_console_modding_now_illegal/

It's meant to ban sale of hardware devices and services that allow playing pirated games on Switch and such, but due to the way it's worded it just bans them all.