I'm going to play devil's advocate. The way I'm visualizing it is like you have one of those wooden gates (https://kiarts.com/wp-content/uploads/2014/10/9-Traditional-Japanese-torii-gate-in-public-park1.jpg) and he scratched his name into the side. If he was allowed to be there (which I assume is true given there was a witness) it was a stupid mistake without ill intention.
Give enough people enough time, and one of them will do something stupid eventually. If the Japanese government is really willing to ruin someone's life over this, maybe you shouldn't open it freely to the public in the first place (instead maybe have a railing or force people to go with tour guides).