The people who are working at these big legacy retailers in 2018 tend to not be very sophisticated about online marketing. I'm being polite with that understatement.
So nobody calls out the marketing departments on this because there is so much political BS going on anyway as everybody is scratching and clawing for their piece of an ever shrinking pie.
I worked in a similar situation and I wanted to stop working with these coupon sites for the obvious reasons you pointed out. I got overruled by higher ups and I later came to learn that my complaints about this practice were a career limiting move.
They just want to be able to take credit for driving a ton of sales even though everybody with half a brain realizes they are not generating new sales. They are simply cannibalizing the business because the vast majority of the time these people would buy at full price anyway if their Google search never turned up a coupon.