Google doesn't like it because they see it as a way that people are playing the system, especially since links are used in it's ranking factors. They see it as buying your way to the top of the rankings, instead of earning it by providing great content that is useful/helpful/valuable to site visitors.
Because it doesn't create too much quality Backlink to the website that will be useful to the improve the ranking the website. if the paid backlink generate quality backlink the google will be happy to that work.