Its Called Affiliate Marketing. You can have a look on Wikepedia, if you want a more in depth explanation in regard to what "Affiliate Marketing" is, but I'll just give you a brief explanation on how it works in this case.
The dictionary defenition is
"Affiliate marketing is an Internet-based marketing practice in which a business rewards one or more affiliates for each visitor or customer brought about by the affiliate's marketing efforts"
What this means is that every time the site you sign up to directs a customer to an affiliate, they get paid a commission.
To get your free gift, you have to be that potential customer and also direct others to be potenital customers too.
The website uses all the commissions to pay for your item and have enough left over to keep a tidy profit.
EVERYBODY WINS!
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