Google is a large corporation who actually fully expects abuse of adwords vouchers to take place. Many people who abuse adwords vouchers end up turning legitimate on them and making adwords more money in the long run. Its free advertising for the adwords program and gives people a free opportunity to fine tune their methods and landing pages, so when they go legit they can push HARD.
ALL blackhatters are NOT essentially thieves. That just shows that you don't understand a lot about blackhat seo. If I create a quality Web 2.0 blog, filled with original content, strictly for the purposes of linking that site to my own money site to flow pagerank and increasing my serp rank, THAT is blackhat seo. I provide quality content which benefits readers, and benefit myself as well. Who am I stealing from?
If he posts a link from his CLIENTS website to his OWN website, he is giving up link juice that could be spread to his own other sites, and he will be pushing his own site higher in the ranks, and since they are targeting the same products, he will be taking sales away from his client. We don't know everything about how Google treats outbound links, but we know enough to say that it would probably negatively effect his site rankings, even if slightly.
But besides all of that, he is abusing the relationship and trust given by the client to add something to that site that he has no permission to do. If I blast comments to some guys blog, it may hurt his site, but he is inviting comments openly from the community. I am not doing it with negative intentions toward his site, and my comments are very generic in nature which could actually HELP him because Google will see more active discussion on the site. The site owner may also believe its a genuine comment and be inspired to keep working harder.