They're not scams, but it's not exactly free either. You have to work to get it sometimes. What happens, is this: Say you sign up for Blockbuster online, for a $10 trial. After 15 days or so of you being signed up, blockbuster sends the free stuff site a commission basically, for helping them add a customer. Now, you must get 5 or so other peole to sign up as well. How they make their money is all of the people who might complete an offer, but never refer their 5 friends. When that happens, they get all of the money and don't have to buy you the free thing. Even when you do get you5 5 friends to complete the offer, they are still making money even after buying your item, though not as much as they could..
It's called a pyramid scheme. The first people to do it, will have no problem finding pople to complete offers, but later on down the line, everybody will have already signed up for it, and it will be hard to find people to complete offers.
Hope I explained it clear enough.. I am drugged up on pain killers right now