When a critic posts an affiliate link for a product they’re reviewing, they’re now earning money directly from the sales of the thing you’re supposed to trust them to evaluate.
That’s a big, obvious ethical issue. And it’s created a snowball effect that has fed the rage, elitism, and skepticism already rampant in dedicated online fan communities.
This is definitely something that raises ethical issues with reviewers. It's very easy to shift into a "salesperson" mindset rather than a "critic" mindset. If your revenue from a review is directly tied to how many people click your link and buy the product, it's easy to slip into a more positive mindset than you might otherwise.
I'm no stranger to product reviews, and I haven't used one on BirchTree in a very long time. I also write for The Sweet Setup sometimes and while they use affiliate links, their model is to write mostly about what they consider the best of everything, so it's not like they're publishing wildly positive reviews of things they think are crap. Also, they let me write a pretty critical overview of Excel for the iPad, which I was very grateful for. There was no pressure to write something that was positive just so they could drive more clicks on the App Store link.