OK, so perhaps reviewers don't exactly "buy" the gear they keep around for reference. But they do keep certain pieces around to compare with review equipment, and it appears to me that they try to keep gear that is particualrly good at one or two things, and/or better than average overall for a particular retail price point, even if this "choice" is colored by manufacturer "discounts".
That said, I don't always agree with reviewer's "choices" based on my (very) limited experience. But it is a "choice" on their part nevertheless. I guess I doubt very much that reviewers universally accept, keep and tout crap gear just because it is free, or worse, incentivised.