In the case of cost per mille/click, the publisher is not concerned about whether a visitor is a member of the audience that the advertiser tries to attract and is able to convert, because at this point the publisher has already earned his commission. This leaves the greater, and, in case of cost per mille, the full risk and loss (if the visitor cannot be converted) to the advertiser.
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.