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

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.