Because it is a undisclosed conflict of interest.Imagine you go with a friend to a bookstore , and he recommends to you several books. Since you seem to trust your friend and you have the money, you buy the books. Unbenknownst to you, your friend receives a 5% commission, and he never disclosed it to you. Same principle. It is a whole different game if he mentions this to you in advance, since now you consider your friend a as an interested part and you will consider his recommendations more carefully.

I would see a conflict if it was selective as to which book. But this gives no preference to one book over another.

So I'm still not getting it. I see no conflict.

So? He can overhype all the books without giving preference to one. He can pontificate about how important is to read books, how all the geniuses at HN read these very important books and so on.He only needs to maximize the money spent on the books, not the selling of a particular one. He is advertising beers (and get money from all of them) not just Budweiser.

Except this page discloses it...

Except it didnt when the post was submitted, it was fixed later by author and good for him since he was in violation of Amazon TOS