I think that if one were to be given something for free in exchange for a review, and if that thing is especially expensive, it will be difficult for him to be critical and honest about the downside of that product.
Pyschologically, one will be somewhat inclined/pressured to show reciprocity back to the giver in exchange for the favor received.
I agree, I know for a fact that I couldn't. Not saying that is the case for all, as I don't believe in blanket statements very much, but I'd say more often than not it happens. Once again, not implying Kunlon or Sorensiim.