The new Google makes more sense then ever. This is how it is thought. The public setting a website are normal, they don't play blackhat (they don't even know what it is). They pay someone for a site and put what they want in there. They don't know what is a backlink. But time to time, they interact with another site putting their link there. They have a backlink.
So it is quiet easy for google to figure out what's OK and what's fishy.
If {search engine} visits site {A} and site {A} shows {backlinks X 0.005} or less than last visit then site {A} is OK. If {content} is updated since last visit then site {A} is {SERP +1}. If {backlinks X 0.005} or more than last visit then site {A} makes unnatural backlink, site {A} is not OK and {SERP - 10}.
That's the idea.