They aren't approving it as a "work of fiction". On the contrary, they are approving it as an acceptable reference book in subjects that include but are not limited to Biology, Astronomy, Geology, etc.
'The Bible is expressly permitted to be used in Idaho public schools for reference purposes to further the study of literature, comparative religion, English and foreign languages, United States and world history, comparative government, law, philosophy, ethics, astronomy, biology, geology, world geography, archaeology, music, sociology, and other topics of study where an understanding of the Bible may be useful or relevant,' the bill reads...
I am not sure how relevant it would be in Biology or Astronomy. But it would be rather useful in comparative religion. Now is it just the Bible that is being approved or any religious textbook?
I don't see any harm in having it available for reference. No religious text should be required reading in order to get any part of any grade however.