You're asking why product placement in retail is different from fake/incentivized reviews?
I don't understand why everyone here is conflating incentivized reviews with fake ones. There are often legitimately good products that we're just too lazy to leave a review, but with a small incentive, we'll overcome that laziness and leave the review. Whereas, a fake review is when you leave a five star glowing review on a garbage product just because of the reward.
While they are both against the T&C etc., fake reviews are scammy, objectively unethical, etc. but incentivize real reviews are not.
ETA: I see now that jj1000 started a discussion while I was typing.