I don’t understand what you’re talking about. I live in Japan, and it’s not true that arrested people have no rights.
If you’re talking about Japan’s high conviction rate, which is 99%, this figure refers to the rate after indictment, not the overall conviction rate.
Also, according to statistics from Japanese Public Prosecutors Office in 2023, the indictment rate after arrest was 30.8%.
Or, are you talking about specific cases?