But I think that's the point - the traffic is effective the merchant's own. They're not driving spammy visitors or buying false visits; they're just redirecting the customers who would have already been browsing the site back to the checkout funnel - they'd just get the benefit of the affiliate tracking code added to the order.
When getting involved with aff-networks I have noticed that there are quite a lot of 'low quality' (of course depending on your needs) networks around that basically lure people into visiting your site, other than referring truly interesting people to you. That seems like business 1-0-1, but it really amazed me how many large networks use shady landingpages with obscure lotteries and other 'prizes'. (This was a about 2-3 years ago, but still..)
Always do a small run first, track what's happening and why and always be suspicious about people 'selling' you other people's data/attention (data-pushers, is what I used to call them) because that's basically what is is, to some extend.