Well the book is separate from the medium, and even just in paper/softcover there are possibly multiple editions available, some with better printing than others. Indian editions are typically cheaper and printed on inferior paper. Regardless I think the common understanding is that the content of the book should be reviewed, not its incidental presentation.

I have several editions of Dune by Frank Herbert. The content is identical in all cases (with the sometimes exception of various forwards); the books differ significantly in presentation. How, as someone who has already consumed the content, am I to know if a particular edition is well made or interesting enough for me to add to my collection?

Maybe you think that is too niche; but Amazon has Amazon-specific versions of some books and also does print-on-demand. Without people reviewing the quality of these things, how am I to know the quality? This also extends to Blu-Ray collections and other things which contain content. Even a Kindle book isn't just the content; how well the book has been formatted matters.

To the extent there is a norm, the norm is that anything related to the thing you are purchasing is fair game; things related to the seller or shipper are not. (Understanding that sometimes Amazon is more than a seller.) If you want reviews specifically of the content, you should go to sites that are specifically for reviewing content.

To the extent there is a problem, the problem is that the star-rating is global for the entire review. I can't give a book 5 stars for content and 2 for book quality. And no matter how good the content, if you're charging me $20+ for a book with pages falling out, you're not getting five stars from me.

I think it also depends on where your review is.

If you're just doing reviews independent of medium, review just the content. The fact that you were reading a poorly stapled together print out from Jimmy down the hall is immaterial.

If it's more like Amazon, where I'm reviewing this particular item I bought, people should know that the item being sold is of substandard quality and they should get the item elsewhere.

I disagree, due to the fact that Amazon is a store that sells books printed on paper. You're reviewing the product as a whole. If you were talking about Goodreads, you'd be 100% right.