Here are a few things I'll add on voucher code sites (and note that my primary site is a voucher code site - codes-vouchers.com). Of course, my general opinion of owners of voucher code sites that are unethical is that they'll probably start using these methods even more.
Things that I've found that are completely unethical -
1) the same thing someone pointed out about not having codes and still asking users to click.
2) having 3-4 clone sites and having them link to each other. this is super easy to find since they have the same name structure with different keywords.
3) using rsspad and then adding a feed for every single merchant they have to get links from a
pr-6 website. there are atleast two sites that do this - a voucher codes site and
4) having forum posts that are nothing except a thousand different permutations of keywords
5) stealing the exact design of retailmenot.com.
6) promoting 15 pounds free shipping at amazon as a discount code.
and so many more.
Of course unethical practices are not limited to code sites - a deals site shows up in the top 5 results has 4 .edu links because they went and found an old unmaintained .edu based forum and added some totally obviously spam comments to it.
Things that I've found that are borderline -
1) the whole click to reveal thing.
2) scraping my blog with wp-autoblog and other spiders.
3) stealing content and codes including exclusive codes - when a code says IVILLAGE how can anyone not understand its an exclusive code.
*** So in an environment like this what is the incentive to be honest? well, to be absolutely brutally frank (and go beyond any one person's ethical standards) both being very honest and being very dishonest are just strategies.
google oughta normally identify and remove clone sites - however, when one or more these affiliates are big google ppc spenders - as a company why would they?