Actually that is exactly what separates good review sites from bad ones. Bad ones are in it to get the free shit. If you want to keep something, that decision should be made after you've ridden the product. That way the review is done already and is as unbiased as it can be.
Only picking Mervin cause they have high value, but if you consistently get like 10 Mervins to review that Mervin does not expect back, and you start giving them bad reviews what are the chances the next year they're willing to send another $5000 worth of boards? Expecting the free product influences your review. Don't wanna bite the hand that feeds.
I come from the tech world, where this exact issue is extremely common.
Nivek's second point is certainly correct and it happens very often in tech. Review sites that consistently give bad reviews to products from a certain brand will stop getting pre-release versions of those products to review. The real danger isn't that the reviewer will give an absolutely glowing review to a horrible product, but that a reviewer will ignore certain minor flaws that might taint an otherwise favorable review. This might not even be a conscious action.
I've studied bias in the context of social science research, and unfortunately the nature of human bias is such that it is often undetectable by the person. Even after my short time here, I fully trust that people like Snowolf and Leo would never purposely give a good review to a bad product. However, it's far more likely that the reviewers might unknowingly leave out some minor details that cast a slightly negative light, after their brains have justified them as insignificant or extraneous. Again, this is not the fault of the reviewer, it's simply the nature of the human brain.
I'm certainly not saying that these reviews shouldn't be done, and I for one thought this NS deal was a great idea. I'm simply saying that it's perfectly reasonable to be concerned about bias in this situation. It's not about buying someones integrity (which I'm 100% NOT concerned about), but rather about introducing bias into their thought process by giving them free stuff.