Why is saying "fiction novel" improper English?

Because by definition, a novel is fictional. It's like calling me a male man, or saying that your pet is a canine dog.

It has one use, though - anybody who uses it in a non-ironic way has a lot to learn before they're capable of writing something worth reading, so I don't need to waste any time reading any fiction they might have written.

It suggests that the person who says it is one of the same people who, when I was teaching, used to turn in papers that began "In Karl's novel 'The Communist Manifesto,' he proves...."

(Note also the fact that this student is on a first-name basis with a long-dead author, and doesn't know what "prove" means. Then there was the kid whose paper had a total of two footnotes, both of them unironically citing "The Onion." Thanks for the reminder of how glad I am that I'm not teaching any more.)

it's redundant

a Novel is Fiction

Anyone who talks about a 'fiction novel' is showing that they don't know what the words 'fiction' or 'novel' mean