Answer: I believe it was because they wanted only the strongest, and the ones who would be the most likely to be able to contribute something to the community.
Step-by-step explanation:
Since this book is showing what extreme communism is, they are wanting to get the pont across that these governments don't value people who don't have a gift. They only want people who will be helpful to them, and to the community. Thus the old, and the newborn who were no longer useful, or were too weak were released.