What's up with all these PTC changing their TOS and making you upgrade?

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I've had about 4 sites suddenly change their terms of service and tell me I have to upgrade to a premium account to get paid. I'm wondering if all of them are turning into scam sites. Why would I want to give them all this money when they didn't keep their original promise to pay me for the ads I've viewed. Maybe I'm naiive, but I have gotten paid from a lot of sites (especially SwatCash, see my profile if you want to check out my referral). But all these new ones I've signed up for recently are flaking out. Some of these are IsabelMarco, SimplyBux, IdealBux, and I'm sure there are more, I just can't keep them all straight. Is anyone else annoyed by this? And are there any sites like SwatCash that are legitimate and actually pay? I've made over $100 with them without having to upgrade or pay any of my own money. Please tell me there are some other good sites like that out there!