Since the emergence of affiliate marketing, there has been little control over affiliate activity. Unscrupulous affiliates have used spam, false advertising, forced clicks (to get tracking cookies set on users' computers), adware, and other methods to drive traffic to their sponsors. Although many affiliate programs have terms of service that contain rules against spam, this marketing method has historically proven to attract abuse from spammers.
Unscrupulous affiliates can squat on domain names with misspellings and get a commission for the redirect. They can populate online registration forms with fake or stolen information, and they can purchase AdWords on search terms the company already ranks high on, and so on. Even if the terms and conditions are clear, an affiliate marketing program requires that someone monitor affiliates and enforce rules.