It's not like reading the story out loud will cause something to happen. Sagume's ability only works on things that can be logically reversed and goals.

But a story with a happy ending might end up having a bad ending instead.

Being Sagume is suffering.

djseifer said:

But a story with a happy ending might end up having a bad ending instead.

But it's a book, so it's not like the last page will magically change into something new. Her ability doesn't work that way.

game2009 said:

But it's a book, so it's not like the last page will magically change into something new. Her ability doesn't work that way.

You're underestimating Gensokyo.

What probably happened is, she said she was going to read them a story, so because she said that, she got so engrossed in it that she neglected to actually read it aloud.