Why PTC Sites are annoying

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I joined PTC sites since June as free or standard member. In fact, the reason why I joined with my current status is to observe how these sites work. I noticed that more of the sites that I've joined need to upgrade or invest in the sites in order to receive pay out. But this condition was never been in the rules and terms of the site. How can these sites don't pay its free members that works for the sites to earn money thru clicking ads?