A Google Webmaster Help thread has a Googler explaining to a disgruntled searcher why spam makes it into the Google search results.
Jack from Google explained that it is hard for Google to prevent all spam attempts on their index. They do their best, but "the internet is a big place," Jack said. Jack from Google said:
We do our best to keep our search results free of spam but the internet is big place and we are far from perfect. We really appreciate user reports which can be submitted via the spam reporting tool in webmaster tools (linked below). Keep reporting issues as you see them and keep creating high quality content; lets try to make the internet a better less spamy place.
To report spam to Google, you can use this spam report.
Google doesn't always take immediate action on spam reports but I am told they read them all.
So Google here is asking users to are upset with their search results to tell them why.
Forum discussion at Google Webmaster Help.