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.
Cashback shopping: Who doesn’t love getting cash back? It's a great incentive for shopping because it feels like you're being paid to do so. It's usually best to use the offers only when you were already intending to buy the product. If you love saving money at discount stores or at places that offer cash back, there are a lot of survey sites out there that will open a lot more offers to you. Survey sites can offer cash back because they're paid a portion of the profit for referring you.
You won't get rich doing these things but it's legitimate and you'll get paid in the end. For example, you get a penny for each email you click. You get a penny for a video stream. You can earn dollars for fulfilling offers but those aren't nearly as quick as reading an email. They are upfront in what you get paid so you can decide if it's worth it.