I think keeping these kind of coupon codes as a %rev affiliate model rather than CPC ads makes it easier to justify the high level of spending (much of which is cannabilizing other marketing channels) - if it was more directly comparable to adwords & Product Listing Ads CPC-based costs, I think more large brands would move faster to cut the wasteful spending going on in these programs.
Right now many rationalize it as "4% of revenue is a great acq cost" - but it's not a true acquisition cost as you already acquired many of these customers and if the affiliate sites didn't exist, 99% of these sales would still happen.
If you're a pure SEO built company, are you really betting it all on SEO? Without it you'd have no company, and if it weren't risky, there would be no opportunity left.