[Borealid]

You can't sell a Harry Potter book, but you can sell any number of Magical School for Wizards books. They can use similar themes to Harry Potter, similar pacing, even similar artwork styles.

They just can't copy the characters, significant chunks of text, or images.

That's the difference between a copyright and a patent in a nutshell: you copyright an implementation of an idea, while you patent the idea itself.