grammar - Why can story writers use くれる when describing a third person?

I just read a famous Japanese legend as follows.

昔、ある所に浦島太郎という若い男がいました。 ある日、太郎は子供達にいじめられている亀を助けてあげました。 亀は「助けてくれて、ありがとうございました」と言って、太郎を海の中のお城へ連れて行ってくれました。

Why can story writers use くれる when describing a third person (亀)?

Answer

「〜あげる」 and 「〜くれる」 make it clear that there are a giver and the beneficiary. 

「〜くれる」can be used to describe an action which benefits not only the speaker but also people who are intimate or familiar with the speaker.

「水泳のコーチが、私に平泳ぎのコツ(tips)を教えてくれました。」

「水泳のコーチが、（私の）友達に平泳ぎのコツを教えてくれました。」

「水泳のコーチが、（私が応援する）佐藤選手に平泳ぎのコツを教えてくれました。」

These sentences are all totally acceptable to native Japanese speakers.

The sentences of the 浦島太郎 story are written from third-person point of view. In general, there are two kinds of third-person point of view: omniscient and limited. I think the author wrote this 浦島太郎 tale in the limited third-person and 太郎 is the author's viewpoint, at least about these sentences.

When a writer uses a character's point of view to write sentences of a story, I think I can say, the writer is intimate with the character at the time. So s/he can use 「〜くれる」 to describe the character's pleasure or fortune brought by someone else.

This 「〜くれる」 generally sounds natural to native readers, because in the story the character is presumably the most familiar to readers too, especially when the character is a hero/heroine or main one, since readers are probably seeing the character's point of view and reading the story with it.

The pair of 「〜あげました」 and 「〜くれました」 is commonly used in old tales (e.g. 『河童のくれた妙薬』) and children's books. It usually shows that someone did a good thing for someone else, and then the second one did a good thing back for the first one. I think it's helpful for children to understand the relationship between 「〜あげました」 and 「〜くれました」, and good because many children like to see both characters happy.