The reason is simple. In most instances the web retailers seriously underpay their referrers. If you have a link to a webstore, you should be getting more money for each sale. If you're a webstore, you should be paying higher commissions. If you've ever wondered why Affiliate programs have become so incredibly popular, you have only to understand the bottom line for the selling site.
Affiliate programs are the cheapest way to acquire new customers - by a huge margin.
Assuming every sale is a new customer, let's compare the acquisition costs with banner ads versus a commission paid in the Affiliate model. But this company understands the lifetime value of a customer. They're willing to take the risk and lose a little upfront, knowing that their low prices and great service will result in repeat sales and eventual profitability. One solitary buck. These figures reconcile with other research I've done.