Google's algorithm already analyze if a link is useful or spam. They can simply ignore the links that are spam and count only good links. But, Google decides to

Google's algorithm already analyze if a link is useful or spam. They can simply ignore the links that are spam and count only good links. But, Google decides to punish the websites caught in the fire. If your website has a link Google does not like, Google can penalize your website along with all the websites that link to you or you link to them.