And if they advocate it for the newborn... how soon until they start advocating it for adults who are diseased, injured, or just plain too old?
I mean
you could say that adults could be killed, potentially, but today, we already euthanaize newborns. We have for quite some time in the medical establishment. We DO and have euthanized the severely injured, sick, etc or those who lose personhood.
Well, I think the relevant types of euthanasia in these types of cases and what you are alluding to would be involuntary and non-voluntary euthanasia. The difference is a bit important in modern bioethics. It is also important to look at the concept of personhood as discussed and defined in bioethics in relation to modern utilitarian bioethics (which is highly controversial, since Utilitarain theory is often counter-traditional ethics and seemingly non-commonsensical).
The primary motivation behind the above is a utilitarian argument. It deals with the application of scarce resources and elimination of unnecessary suffering--both of which are an inhernet part of modern Preference Utilitarianism.
The target of the ethanasia in this case is, I think, only those who are suffering or suffer from disabilities that would lead to significant suffering, especially if the newborns will have roughly short lives filled with suffering. According to Utilitarianism, it is wrong to allow someone to suffer when the probability is high that that individual will have a short, painful life with little capacity for a quality life. We already allow "killing" of infants when doctors and parents decide their child will generally have more suffering than happiness or when the child is unhelpable and will live lives with suffering. They just do it through passive euthanasia, but the consequences are little different according to Utilitarianism. Extending it to active euthanasia isn't really all that objectionable.
From much of the bioethics literature I have read on the topic, the heated issue is personhood. Most bioethicists, and especially those who stem from P.U. Bioethics circles, define personhood differently from the deontological ones. Fetuses, like early newborns, would fall into what bioethics calls non-voluntary (not involuntary) euthanasia. It is non-voluntary, instead of involunt, because fetuses, like newborns, cannot choose. They cannot consent. The literature is contested, though, so I have dificulty, but most seems to content that fetuses and early newborns are not moral persons in that they don't have the key rational attributes that make a moral self that we see in children or adults. The same can be said of some adults who lose the rational faculties, but not all sick or ill would be. Many would be persons according to the modern bioethics definition.
They don't see it as morally as bad to kill the fetus or the newborn as it is to kill the 12 year old or the adult because of the personhood issue. Killing someone who had moral personhood (a self concept, rational attributes, autonomy, preferences for the future (including life)) would be different from killing a newborn because the adult (whether sick or what not) has attained moral personhood via its characteristics. It would then fall into involuntary ethanasia, which requires the active thwarting of defined passive or active preferences for future actions, including, but not limited to, the desire for continued life. Thwarting their lives thwarts more preferences, and the idea behind P.U. is to maximize their welfare preferences.
It's a bit complex what they tend to argue, but I will try to summarize it.
Many of the bioethicists see fetuses, like the newborns, as replacable utilty recepticals. This means that, since they are not persons as per their definition, it can be ethical to kill them if it is to minimize suffering, since those types of beings fall into the classical hedonistic utilitarianism. They are not persons, so they lack the complex preferences, desires of person-hood consideration. It is ok to kill a severely disabiled infant, according to them, if and only if doing so will reduce the net suffering in a child who will likely have more suffering than happiness.
They also advocate it if the receptical dying will lead to another child, who otherwise wouldn't exist, and would likely have superior life-prospects (less suffering).
Resource allocation isn't the primary concern, but it should be considered. It is a waste of resources to shower funds and supplies on someone who:
A. Has no personhood
B. Won't live long
C. Will likely suffer greatly or have more suffering than happiess
D. Whose existence will cause significant burden to the family prior to expiring early (while suffering).
E. If the resources for said individual can be reallocated to create greater Utility.
A key tenet of utilitarian bioethics is that medical resources are scarce, and it is morally good to allocate the resources to those who can benefit from them most, as in, where they are needed most and will procure the most utility (minimization of suffering and increased life quality). If a terminally ill child is suffering tremendously, and will likely not survive long, but will be a big burden, it is better to kill him and redirect the resources to those who will survive but can be cured, helped, and will be able to live relatively happy, healthy lives.
The problem itself is that it is inherently a part of Utilitarianism to consider resource allocation issues in medicine; modern utilitarianism often conflicts significantly with current medical ethics, which are actually more deontological.