I don't agree with what they did because its very extreme and painful for their daughter.......... But I can see where they are coming from.
I have seen many cases where the disabled adult is left in their bed all the time, they are bathed and fed in their bed because moving them is a burden. I think that in the case of if they both died, they wanted to be sure that their daughter would be "easy" to take care of so that she would not be neglected. (i am saying "if they both died" because they stated that they were not doing it for it to be easy for them and that if she was 300lbs they would still find a way, so it has to be in the instance where someone else has to take care of her)
the thing that they don't realise is, regardless of how lite and uncomplicated ( no menstrual flow, no worries about breast cancer or appendicitis) they make their daughter the fact remains if the caretaker views her as a burden then she will be neglected. So basically they are putting their daughter through all of this pain for nothing.