$1/20k gets shouldn't be a big deal?

So much spam would go away if we just charged $1 per 5000 emails. Normal humans would be fine. Mailing lists would need charge, but it would be minimal for most useful mailing lists.

Maybe Amazon is going after some fraud?

My theory is they see POST/PUT calls as adding value and are actions that customer need to perform and help the platform (adding products, inventory, etc). But GET calls they see as leaching off them and not core requirements.

The huge gap in this theory are things like Seller Fulfilled orders which you have to GET, but for Amazon they have always pushed hard for Amazon Fulfilled instead.

In the end I think this is all mostly to keep things/control in-house as much as possible. Some apps will shut down, others will pass the cost along the sellers. Sellers always lose.