The base fee doesn't need to be monthly, they can take $50 dollars as a one time registration verification fee. That should be enough to cover their compute costs for the year, specially when you pool in that from multiple customers. Who is spamming with 100 monthly emails? How much compute do they need to verify you aren't spamming. They can bake all of that into the pay per use price, they choose not to and I'm glad cloudflare is offering this.

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.