This misses the point. If you have the public domain ebook already, Amazon isn't knocking on your door asking for their cut.
Getty literally told the photographer she had to pay a license for her own work posted on her website. She didn't acquire it from Getty.
You can acquire a public domain ebook from somewhere else for free or you can pay 99c to acquire it from Amazon. But either way, you're not paying for a license to copy it.
That's the rub in this case it seems, that if Getty never took this action against the creator herself, who understood the situation and, I'm not sure they'd run into much stink about selling Public Domain images as a business component (they provide a service). Her catching them in the act is stunning.
Getty literally told the photographer she had to pay a license for her own work posted on her website. She didn't acquire it from Getty.
You can acquire a public domain ebook from somewhere else for free or you can pay 99c to acquire it from Amazon. But either way, you're not paying for a license to copy it.