Wondering about PTC sites

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Hi, I've just got broadband connection lately and tried to look for some home based jobs then I found these PTC sites who are claiming to pay me just for clicking the advertisement. I know that it's part of online marketing but how can they pay such a large number of people. I know that some of them are scams and asking for membership fee and asking people to buy items for them. Are they legal? What are they for? Who are paying them? How about those paid to answer survey sites?