Google will figure the higher bounce rate of the page (the user quickly went back to the SERP) means it's not as relevant as they thought, so they'll move it down in the search rankings.
The Pinterest issue has been there for 4+ years. When does Google figure it out?
Maybe it's not an issue for everyone? Maybe the bounce rates on average aren't that bad and some people are spending lots of time there? It's possible, but I agree they aren't very user-friendly in search results for someone who doesn't have an account. I also avoid them.