Oh the Irony
"We expect that advertising funded search engines will be inherently biased towards the advertisers and away from the needs of the consumers." -- Google cofounders Larry Page and Sergey Brin, 1998
Google, as a result, made a separate, distinct location for advertisement links. Because they separated the location of advertisements from the search results, they preserved the integrity of the search results.
This structure has not changed, as you can clearly see sponsored links are separated from search results: http://www.google.com/search?hl=en&q=pizza
"Hey Sergey, take a look at what these guys at Overture are doing..." - Larry Page