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.
I tried one survey company (don’t remember now which) they sent a reasonable amount of them my way, but I never qualified to take the entire survey…either I had to answer that someone in the household was gluten free or that I didn’t have senior pets or some other random question, and it would kick me out and tell me thanks, but you don’t qualify….got discouraged and gave up 🙁
IOCS is a not-for-profit research organization that conducts studies and experiments on shopping-related behavior – things like how we, as consumers, evaluate products, how we make the buying decisions, etc. Although the focus is mostly on shopping related behavior, some of their experiments and studies include broader areas of marketing, psychology, and economics.