Art and literature should be moralizing because they are powerful forms of creative expression that can bring about significant moral change in people. It also imparts moral teachings that are crucial to such forms of creative expression.
There is no such thing as amoral literature
The majority of novels contain some underlying philosophical, political, or social principles and it is impossible to create an amoral narrative.
Art and literature should not be moralizing because that is not the goal of the medium. Additionally, they usually fail to be moralizing for every viewer or reader. The viewer often ignores the artist's intentions.
Objective morality does not exist
Literature and art should not, and can not, be moralizing because objective morality does not exist.