Morever, Goodreads doesn’t just cover a few popular fiction or non-fiction works for which a short multiple-choice quiz could be realistically created. It allows you to catalogue and review any book that has an ISBN. As someone involved in an academic field, my Goodreads activity is mainly obscure works of scholarship from university presses.
As far as Amazon investment goes, it seems like a missed opportunity if they're not putting at least some more money into it. That's got to be the best possible marketing platform for their ebook/kindle and wider book sales.
I'm always happy to get those if I liked the earlier purchase. Unfortunately, I've bought some kindle books that turned out to be not so great, and there doesn't appear to be any way to exclude them from this feature.