This is a very interesting debate. I don't have a defined opinion on this matter, but this specific situation brought some questions to my head.
First: let's say the traffic trading part is all cool, but this guy was loading viruses, which he never denied (he simply evaded an answer on that).
Second: Let's say we don't want to hurt people's feelings and we call it "spyware". Now, why somebody would want to install spyware in their surfers' computers? To make something good?
And back to the sponsor point of view, what if the installed spyware is the kind that steals signups? Then my content would be used not to promote my site, but to avoid that the surfer EVER signups to my site.
Third: Traffic costs money. Let's say a 100k thumb TGP gets 30,000 clicks to their trades per day, 900,000 a month. With very inexpensive GOOD traffic, that would be at least 3k per month, paid by sponsors content, with no reward at all
Fourth: If surfers click on my thumb is because they wanted to see my content, right? But then they see that the thumb with my content is just a blind, making him click another thumb or go to the new TGP where he was redirected. This way, the thumb TGP owner is actively working to steal traffic (hence a possibliity of sales which by the way means money for the TGP owner) from me at the same time he makes profit (see third point)
Fifth: why the TGP owner simply doesn't buy a couple sets and use only thumbs from those sets and everybody happy?
I had more question, but for now I think this is the more important aspects I could find