Media mail is a way to make the USPS subsidize schools, teachers, and students. Fiction books are used in English classes so they are considered educational. They set the rules in a way that's enforceable. Imagine if they required you to show ID to prove that you're a student or teacher, or the clerk had to make a judgement on whether or not something is educational. It's easier to consider books all okay for the discount and (magazine shaped) comics not okay.

This circuitous way of subsidizing education costs reflects America's aversion to taxes and cash transfers. The USPS does not take tax dollars, it's entirely funded by postage (electric vehicle funding is an exception). Yet it is expected to forego revenue for mostly public education services. Teachers must front the cost of some school supplies which they can deduct from their Federal taxes (up to a $300 limit). Other "It's not a tax" tricks include shifting some city services to the HOA level (HOA fees are not a tax), requiring private businesses to provide free or discounted service to the government and low income customers, and raising user fees for government services.