You were still willing to pay the listed price. The fact that you were subsidizing their reward program doesn't diminish the fact that it was cheap enough for you to buy.
Also they maybe doing this promotion as a loss-leader to build marketshare/brand recognition and are willing to work with lower net profit.
For example, the top search result for “webcam” on Amazon.com right now usually sells for $35[0]. Its wholesale price on Alibaba for 5k units is $3.84[1] + $2.04 shipping per unit. Plug those numbers into Amazon’s FBA profitability calculator[2] and you’ll find that the seller only needs to sell 67 units per month to break even, after giving every buyer $20, because the seller’s net margin at this volume is nearly 60%.
[0] https://camelcamelcamel.com/product/B087NN41JH
[1] https://www.alibaba.com/product-detail/China-Factory-Wholesa...
[2] https://sellercentral.amazon.com/hz/fba/profitabilitycalcula...
The basic idea being that you share a handle between all 3 places, so it's easier to determine the trustworthiness of the reviewer -- both in terms of identifying fake/paid reviews and figuring out whether the reviewer's standards match your own.