The actual law literally does make it illegal to mess with saves if they are protected.
I finally took a look at the official english translation.
What it pretty much does is apply their equivalent of the DMCA to game saves, which until the amendment in 2018 were not covered.
Note that getting said modified saves from outside of japan doesn't dodge the law, as it specifically includes importing.
This arrest was one of the things that the law was amended specifically to allow jailing for.
It also catches save exploits used for softmodding/execution of unsigned code. Soundhax is still MAYBE not blocked by it because the sound player program is intended to read user supplied sound files. For the same reason free dvd boot is not directly prohibited by the new change, because a user supplied burned dvd movie is intended to be processed by the console's dvd player. But the law still gets you, because you are using it to run unsigned code, which bypasses the restriction that tries to prevent you from doing it.
The 2018 changes were intended to, and do, close loopholes that were in active use. People were saying "it's just a save game, those aren't copyrighted, they don't count, you can't touch me" and it was working before the change.
Note. what counts as a protected save is very broad. A save being inside a cart counts, because no interface is given to access it directly. a save file on a PC doesn't automatically count, though if cryptographic signing or checksums are involved, then it DOES count. The general rule for modifying a PC save is that if you can make arbitrary changes with a hex editor, and have the file still be accepted if the format is valid, there is no technological measure preventing it, and you can do it legally. Validating the structure of the save file in itself doesn't count either. But if there is any attempt to specifically detect the file being changed and prevent it from being loaded, then it counts.
If a game reads a graphic file and use it for a texture and you can simply replace it and the game loads the changed texture, that's fine for an installed PC game, because there is no technological measure preventing it. Drop in a different model? Just fine, as long as the game will accept it. PC game mods that are intended to be possible are fine. Even if not specifically intended, if there's nothing really stopping you from it besides getting the format of the file right, it's fine.
A clarifying example is the original DOS DOOM game. Even before PWADs existed, the game accepts any valid iwad that conforms to the structure expected. So you are allowed to make your own. iD software added pwad support, to make modding more convenient, but it was not originally intended when the game was made. There is no technological measure protecting the iwad file. The unregistered version, however was designed to only take the iwad it comes with, to stop people from getting the whole game without paying. This version DOES have a technological measure protecting the iwad. Pwads that run with the shareware iwad are fine. But making say an iwad that effectively converted the shareware version into the registered version would be possible to prosecute under the law.
The guy got in trouble because his actions were provable. If you just do it for yourself, it's very hard to actually get proof, and prosecution is thus difficult.
It is unlikely that the law will be used to prosecute anyone against the spirit of the law. Prosecuting someone for modding their single player Skyrim is not within the spirit of the law. This use of the law was definitely within it's spirit.
Source: Asahi News - Translation Courtesy of Serebii
