Using tricks at the detriment of the user to the benefit of the service is blackhat. Pinterest is doing these tricks to their benefit at the extreme detriment to everyone but themselves.
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.
