I do not consider buying positive reviews with free items an ethical marketing strategy: it deceives people expecting unaffiliated opinions there...

Why should anyone take part in this?

If a company decides to give away free items, and gets nothing in reply that's their problem... I am happy when people do not sell their kidney for pennies: and as a single costumer you usually don't have the power to fight back large corporations: then sometimes the most convenient reaction is to keep both the free gifts and your kidney ;) And if many people do this, it may eventually get them out from business, win-win

edit: Thinking over, I partly agree with the previous comment. If they expect you to update your review, honesty may be the most ethical, so e.g. one can start it with: "They have sent me a free item after I returned mine and gave them a 2 star review..." ;)