Earlier quoted context omitted.
I think seeing all the random items is collateral damage. It can possibly be fixed (only show purchases in the category that you referred?). As the affiliate, the insights are invaluable. 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. Or if you saw that they ended up…
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.
The site owner only sees that 35 people bought book X while 24 people bought book Y.