It's weird to blame Pinterest for Google's choice to promote the site. Sure Pinterest wants to rank highly, but they aren't tricking Google. Google knows how popular they choose to make Pinterest results, and how much most users like those results.
If I make an algorithm to rank sites, and you figure out the internals of that algo and manipulate your behavior to rank at the top, that's bad and not the same as being Google's choice, at least not directly.
My friend, Google tells you how this works. Pinterest doesn't have secret knowledge.