Well, Google is doing all sorts of things to generate revenue, doesn’t matter if it’s coming through wicked means. Today, Google is not limited to just one business; it holds variety of products and services that has indeed opened doors of revenue.
Relevancy for Google hardly matters, revenue comes first. So why not mix paid results with organic results. Cloak top results with sites that are Google’s allies or long term partners. We have seen Adsense themed sites cloaking the top ranking with no objective to stay in the rankings for long time but Google is hardly concerned about these sites holding top slots because Google gets money when some one clicks on those Adsense ads. It would be better to say that Google can’t disturb its Adsense allies.
Also, the spam results at the top on major keywords are definitely boosting the Adwords popularity among advertisers because they have to survive and compete in the business. What will you do if you are not getting position in SERPs despite sincere and ethical means? You have no other option but to resort to Adwords.