Remember that World War II had recently ended when The Chrysalids was published in 1955 and John Wyndham served in the British military forces. The code of war is to endeavor not to leave a single member behind. You know from all the national and foreign natural catastrophes that have occurred in your lifetime that all possible resources are pulled together to rescue every last individual--or find every last individual.
John Wyndham applied this ethos (underlying social belief) to the story of The Chrysalids by having the other country, Sealand, come to the rescue of a handful of young people. It may seem strange when looking from outside-in through the porthole of the story--a simple story, in some ways--but in reality, the rescue by Sealanders of people with shared humanity who were in danger and cried out for help rings true with myriad examples in history. Look up the U.S. rescue of Cubans on rafts in the Straits of Florida between the Gulf of Mexico and the Atlantic Ocean (1994): This was propelled by the same ethos.
I'm not sure the answer above is correct since the Sealand woman doesn't deem it 'worth it' to go back for the other thought-speakers. No, I believe that they went through so much trouble to advance themselves with Petra. They weren't interested in David or the others; as she says herself, they would never go as far as Waknuk to rescue someone, but it was different in David's case BECAUSE of Petra. Also, when she meets Petra, she says 'It was worth it! At her age and untrained-yet she can throw a thought halfway across the world! She still has alot to learn but we will give her the best teachers and she will eventually be teaching them!' (pg193)
So you see, it wasn't because of the author's military training to 'never leave a man behind'. It was simply because they wanted Petra to advance themselves.
Hope this helps