| I'm not sure I'd go so far as to say it's an outright scam, but you can guarantee there's a catch somewhere along the way. They more than likely gather your contact information and sell it to companies for fairly high prices.
When you sign up for things like this, remember that registration implies agreement to the terms of service and the privacy statement. Many companies actually have a clause within both about the handling of your personal information that makes it legal for them to share that information with "select" people. | 
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