If she was there of her own volition, there would be no need for guards, and doors, and locks, and for people who took her away to have taken her away.
She would just walked into the prison when she wanted on her own terms. (unlikely)
It's only voluntary if the doors are unlocked, the guards are gone and you still chose to be there. The whole reason she's there is because they want something from her, it is a legal form of blackmail.
