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 see how that can happen. It's not even wanting to keep the board for yourself, but getting more boards to review for the next year to keep the site going?
What a flawed system. Industry standard should be, send the damn board back.