The coupons that you're talking about are for specific products. The idea being that you come in to get that price on one thing, and wind up picking up others at full price.
Some stores offer coupons for your next purchase. For example, Gap gave me coupon for 20% off any one item if I complete their online survey. That's a pretty big discount, so that survey information must be pretty valuable.
I think that they don't expect you to come back for just 1 item. So I would guess that they value the engagement and increased chance of a trip to a store.
I don't know their margins, but I doubt a 20% discount has them losing much on that item.
