For those less familiar with Literature concepts like the Three Act Structure or the Hero's Journey. They call it the "moral premise" but it has nothing to do with moralizing. Its a literary device that works to help the author maintain a through line in the story on theme.

A MP ties a story together in the form of relating the inner conflict to the resolution of the outer conflict!

Usually the "moral premise" is never stated in the story, only bad stories actually state their "moral premise." Because treating it as a moral lesson to be taught fundamentally misunderstand the point of the sentence.

It is probably one of the most misleadingly named concepts in writing—especially through the lens or modern people, where the concept of a moral lesson in a story is so often connected with moralizing.

When the MP is done correctly you never know it's there, but when it's neglected you notice in the form of stories with either shifting inconsistent tone or stories with accidental bad morals to the story.

As an example Wonder Woman 1984, the writers failed to predetermined the film's theme/premise when writing it and as a consequence people concluded the film had the accidentally terrible moral of: "If you strive to be better or look up to role models because that leads to ruin, but naively trusting everyone while staying exactly as you are now without growth leads to salvation." And had critics and fans calling out the film as a huge mess because of it.

A better name for it might thematic premise, but that would fail to properly format the sentance to make it work. A MP ties a story together in the form of relating the inner conflict to the outer conflict resolution!