Why do paid links violate Google’s guidelines while other ads don’t?

Google gives penalties to sites who do paid links, but doesn’t penalize site with other ads such as AdSense, Chitika, etc. Why? They both are source on income to a site, a site needs money to grow or sustain. Net4earning, India Google’s guidelines on link schemes: support.google.com Have a question? Ask it in our Webmaster Help Forum: groups.google.com Want your question to be answered on a video like this? Follow us on Twitter and look for an announcement when we take new questions: twitter.com More videos: www.youtube.com Webmaster Central Blog: googlewebmastercentral.blogspot.com Webmaster Central: www.google.com