No one doubts that it's useful for the people standing to make money off of this. I think the question was whether it was an invasion of the reader's privacy, who probably don't realize that the blog owner can now track their buying habits.

Only in the aggregate. The site owner doesn't know who purchased what. Only Amazon (obviously) knows.

The site owner only sees that 35 people bought book X while 24 people bought book Y.

Anonymous data can easily become identifiable when it's small enough, as GP was saying.

If someone runs a tiny blog that they attempt to monetize with affiliate links, and one of their only readers at the moment is their mother, I don't think they'd be happy seeing that one of their readers purchased a sex toy.

This is exactly the right approach. Anything specific enough can be traced back to an individual.

Why not ask people for this kind of stuff in an honest[1] way? If that doesn't work well because people rarely want to share that information, collecting without asking doesn't seem right.

[1] By "honest", I mean like a pop-up with a question or two and a clear "Not now" button. Not another cookie banners or passive collection.

> * Imagine you hosted a sleep blog and directed people to a particular memory foam pillow. If you saw they ended up purchasing a different pillow, you could change the link to better serve your readers.*

If I buy a memory foam pillow from company B instead of company A then I don't want company A to know at all. If I wanted company A to know then I would tell company A why I didn't purchase its product.

Maybe it was that I didn't see company A's product. Or maybe it's because company A is a piece of garbage. Maybe it's because company A employs deceptive practices and I don't want to give company A my money.

In that example company A isn’t the one who sees it or gets money. It’s the third party sleep blog who you just clicked a link on for “top 5 memory foam pillows”. Now they see which one people buy most and can give that the favorable rating

Shouldn't the blog be giving the favorable rating to the pillow they think is the best, not the one that's most likely to sell through?

And then what does that say about the written article? It's just metrics driven for what is the "best". It's truly a terrible system that's taken place over the last 20 years or so.