They include sites that seem to be their competitors because they earn referral fees when you buy memberships. A couple mentioned in scam forums even try to dupe you into buying the same list at other membership sites they own under different names. Naturally, these sites also have an incentive to exaggerate how much you'll earn from online paid surveys.
Companies, brands, and organizations from around the globe are constantly seeking the opinions of people just like you to help shape the products they develop and how they market them. They count on paid online surveys to provide them with reliable information. If you spend time online and enjoy giving your opinion, paid surveys are a great way to earn a little extra cash while helping these companies.