All of those options are true, but you forgot the forced confessions. 30 days detention before charging is long enough for the bruises to heal. On the other hand, they prosecute far less crime across all categories than in the United States.

The staff of prosecutors is much smaller in Japan, so it likely means they mostly prosecute the winnable cases. In contrast in the US, a district attorney might choose to prosecute cases which make themself more electable.