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Google works hard to prevent other webmasters from being able to harm your ranking or have your site removed from our index.
Ignore links from bad sites, period. They don't count, they just disappear from the link graph. That would put an end to link buying, and negative SEO.
If this is publicly known, this is not a good solution. Spammers would just threw billions of links knowing good ones will stick and bad ones will be ignored.
Google has to remove the ability for a third party to influence a site they aren't in control of. Until they do, Google's SERPs are compromised and do not represent the best pages for a search query, nor do they provide a good user experience.
Just looked at the updated page - "Google aggregates and organizes information published on the web; we don't control the content of these pages."
HAHAHA - so WE can control the content of OTHER people's sites!? That has to be one of Google's best comments yet!