Just because there is a cure for something, doesn't mean that someone still can't die from it. There is a lot of treatment out today that cures things that will kill you yet in some cases people still die from it, usually because they can't get the cure from where they are.
Amanda might have been unaware of the cancer until it was too late, or maybe she wanted to die. We have no idea who she was and how she lived at that age and that is assuming Burke didn't give Ripley false information.
