Some people still don't accept IVF because it isn't "natural". Some people don't accept vaccinations against horrible diseases because it isn't "natural".
I don't understand/can't imagine this woman's motives either, and they do seem to me a bit "unnatural", but I don't feel my understanding of other people's motives is really necessary if they are not doing any harm. I feel confident that she is not doing this in order to cause harm and harm may not come of it even though the odds would appear to be stacked against her and the babies. So why should I object? Saying "you can't do this because I don't like it, don't understand it/you, wouldn't do it myself, think you're bonkers for wanting to do it" is really rather puritanical in my view.
jess's objections based on a possible overuse of limited medical resources are most reasonable I've read on this thread, but even they are not cast in stone. People with various medical conditions are always pushing at the limits of medical provision and knowledge. We don't usually blame them for it. The only difference here is that this woman has chosen to be in the position she's in; I guess most people don't choose to be ill but quite a lot of medical conditions are brought on by bad life choices (e.g. smoking) that people have made, against advice.