People will jump on it and vouch for your site as they get paid. Other people will see that as a positive sign and will start buying these hash contracts as well. It's literally that easy.
Problem is that people are desperately looking for ways to double their money within an x number of weeks or months. In order to achieve these returns they will take every risk for granted.
people are still falling for it even with all the warnings though. if they do, i think they deserve it as they are all helping the scammers to scam. its a motivation to the scammers. these people are in a hurry to get money back. in fact they always wanted to be the first to send btc to these doubles in hopes to be the first to get paid and put testimonials as well.