This has nothing to do with unwillingness to exploit. If you would get a 1 million dollar coupon by leaving a 5 star review, almost everyone would do it (except perhaps billionaires). The thing is: Everybody has a price, but what that price is, is always different.
The less money I have, the more incentive I have to take advantage of these schemes. For me, the effort of earning 20$ this way would make this a few hundred dollars net loss (compared to working my job instead). So yeah, give me a 1000$ coupon and I am game!
For the same reason, I rarely return items I buy if they are in the under 50$ category, or write negative reviews for products that suck. It's just a waste of time, considering time is the most precious resource, and it is also money.
I often write positive reviews for products that amaze me (doesn't happen too often), but here the product gave me so much value that I am willing to spend my time for this.