I'm only making a guess at "strong proportionate reasons", but I would say that it's similar to "do not resusitate (sic)" orders, which aren't uncommon with adult patients who are terminally ill. Life may still be maintained, and it clearly depends on who is giving the "order" (patient, family); a newborn obviously cannot communicate on his/her own behalf, and the Church may simply be saying that life should not necessarily be preserved at all costs. It's not a matter of whether a life is worth living, but whether a physical life can be maintained without extreme measures. We have wonderful modern medical technology...is it always a blessing?
See #33