Some say Google doesn't improve its search results by removing spam pages because Google earns money from the ads that run on some of these pages. Others say their websites were removed from Google's index (or penalized) to buy AdWords ads.
But Google's co-founders, Larry Page and Sergey Brin, wrote something even harsher in The Anatomy of a Large-Scale Hypertextual Web Search Engine (1998):
"Currently, the predominant business model for commercial search engines is advertising. The goals of the advertising business model do not always correspond to providing quality search to users. For example, in our prototype search engine one of the top results for cellular phone is "The Effect of Cellular Phone Use Upon Driver Attention", a study which explains in great detail the distractions and risk associated with conversing on a cell phone while driving. This search result came up first because of its high importance as judged by the PageRank algorithm, an approximation of citation importance on the web. It is clear that a search engine which was taking money for showing cellular phone ads would have difficulty justifying the page that our system returned to its paying advertisers. For this type of reason and historical experience with other media, we expect that advertising funded search engines will be inherently biased towards the advertisers and away from the needs of the consumers."
Google doesn't accept money for a better ranking, the sponsored links are clearly labeled. But it's interesting to see that even Google's co-founders didn't trust the soon-to-be Google's business model.