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

Uploads by GoogleWebmasterHelp 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:
http://support.google.com/webmasters/bin/answer.py?answer=66356

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