Seems to me that pregnant women in prison are in no position to give their consent to the procedure, much the same way that they are not considered able to give consent to have sex with the staff. The power and fear dynamic in prison is way too strong for it to be ruled out. Even if the woman technically consents, even if she thinks at the time that she actually consents, the conditions just aren't there for us to be sure she truly made a good informed decision for herself of her own free will. It's not like the outside world where you have a ton of choices on doctors, so if you don't want to do something they suggest, you don't have to worry that you're going to make them mad or make them like you less and that you'll be stuck receiving bad care or being denied care you need. I know I'd be afraid to upset the doctor who was about to be in control of something that could very painful or dangerous if he opted for it to be. And they don't have access to plenty of second opinions and good reliable information about their health situation and the risks and rewards of sterilization or future pregnancies. They're stuck with relying on one doctor to tell them it's a good idea or a bad one. And apparently, he may have interests that aren't strictly related to the mother's health, like the one in the article that was obsessed with mothers being on welfare.
It might fall under empowering women and offering them a service that's routinely available to women who deliver their babies when they're not prisoners. But for inmates, no way. It's depriving them of a chance to make the best decision possible about something that they can never undo.