That's a good point, however considering that there are many thousands of books one might choose to read, by your logic the HN front page ought to be filled with "add to cart" links for those books on Amazon. Frankly even Amazon lets you read a few pages of a book and reviews before deciding whether to purchase.
I think the more important question is whether the HN home page should be directly supporting revenue generation efforts by media orgs. They contrive article titles to be clickbait, only to load a paywall when the user clicks with absolutely no context, no preview, etc.
This is about HN taking a stand against being used simply as a revenue generation "channel" by media orgs. I'd be hard pressed to find a more intelligent, wealthy, and influential demographic than HN readers. News orgs have realized this and they actively promote stories on HN to drive revenue.