Maintaining the reputation of an IP address is the issue here. If one bad actor sends just a few emails that get marked as spam, the entire IP gets marked as bad. That's basically what you're paying for.

Also, the person who just wants to send a few 100 emails a month is actually far more likely to be a spammer. So it's also a way for them to eliminate those folks.

And lastly, the support burden can be high.

AWS has basically said they only want serious customers, let the other guys worry about the small senders.