20% fee isn't the whole story - about 5% is the cost-of-fulfillment - website, phone support, CC processing, fraud protection, etc. The rest of the "fee" is variable marketing and promotions costs - higher search placement, "buy one get one" offerings to customers, etc. The calculus for the restaurants is whether they spend any marketing budget, and if they do, is it going to be junk-mail flyers or targeted advertising that they have some hope of tracking success with.