That was pretty crafty. Make it voluntary and thus when she's eventually declared not guilty, they owe her nothing because she was voluntarily agreeing to be under 24/7 surveillance in a closed-off facility. If they had instead arrested her, keeping her locked away for too long could have been more problematic. I'm sure even Japan, with it's police yakuza, has some laws dictating how long a person can be kept in a cell without actually formally accusing her of anything and judging her guilty in the court.