Google are refusing paid links. Because paid links can change search engine result page.

Why? Because paid links is just like another natural links. So the googlebot can’t read it as advertising, but it read as normal links and acting as a vote.

This behaviour is abused by website with high PageRank. They sell pagerank, not traffic. So google is not agree.

Just read link below to have better understanding
http://searchengineland.com/070420-111550.php

Popularity: 5% [?]