Answer:
In my opinion, I think it's good that the authors make the reader infer the theme of a story. Each reader is different and may have a different view of the story than other readers would. And also, having young readers infer story themes can help expand the reader's imagination and thinking ability, rather than if the author just TOLD the theme of the story, then the young reader wouldn't be able to hypothesize and use their imagination while reading.
So in conclusion, having the readers infer the theme, helps the reader to come up with the big ideas themselves, using their imagination--making reading more enjoyable and fun.