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.
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.