I know that some people consider things a scam simple because they didn't earn money the way they anticipated. If all you do is surveys and you get denied on all of them, it makes sense that people might assume it's a scam since all their work didn't earn them anything. What they don't realize is that they aren't trying to earn with the other methods. It doesn't matter that they got the $0.15 from the PTC, the surveys didn't pay out so it's a scam in their minds.
Some of it is the fact it isn't instant payout, or that it's a high payout, or that you need $15 in offers to get a payout so they can't just refer a bunch of people and not work to get paid. When I first tried to cash out, I think one of my referral bonuses was denied, dropping my money below $25, and thus denying my cashout. I could've assumed it was a scam based on that cause they found some strange reason to deny my cashout. Instead I kept going and realized it was just something that I had to deal with on this site.
Some of it is also that you have to put some effort into this site. One of the sites I'm on can earn 20 cents a day and the minimum cashout is 60 cents instantly to Paypal. So waiting a month or two for enough to hit the $25 limit is hard for some, even worse is the fact you won't get paid till a month later. Another site I joined just raised the limit to cashout to $2 instead of $0.10 and people are complaining about how it's going to take forever to earn that much. So, again, imagine waiting for $25 instead of $0.10.
Some of it is they would rather believe it is a scam instead of believing they are missing out on something that could be earning them some money. I earned about $100 in a year here. That's not a ton, but it beats nothing in my mind. Think of how many people could use an extra $100 a year for doing a few simple things they already do online.