but I need to sign up for Pinterest to have access to the link

I thought Google had guidelines against deceptive cloaking.

What happens if you change your user-agent to a Googlebot one?

...

That kind of thing isn't unique to Pinterest. I'm constantly landing on intriguing articles from reputable scholarly publications ... only to find that I, unlike the Googlebot, have to register to read beyond the abstract.

That's a pay wall. I get that.

For forcing membership to view free content... that is available for free on other websites? That's is really irritating.

Here is a quote from Google [support.google.com...]

Cloaking refers to the practice of presenting different content or URLs to human users and search engines. Cloaking is considered a violation of Google’s Webmaster Guidelines because it provides our users with different results than they expected.

In this case the user would expect to see the content that Googlebot crawled, content that was subsequently presented in the SERPs as a potentially useful answer to a particular search query.

Pinterest requiring registration would seem to be a clear violation of the guidelines.

And Google is providing a useless response to a search query.

...

That may become Pinterest's downfall: Having to register just to view. I can imagine registering to carry out actions, but just to view.
I tried it and a popup appears asking me to "sign up to see more about this Pin" and there was no obvious way to close the box, I had to close the window.

Pinterest is locked behind a registration requirement, it's no longer a useful thing for Google TEXT search results. On the image front however I highly doubt Google will want to remove Pinterest images from their results, especially since Google displays full images right on their own domain without sending the visitor.

My take - Pinterest and Google both need to stop subverting website images for their own profits at the expense of the owners. Send visitors easily or do not take the images, period. Google is guilty with their full gallery displays and Pinterest is guilty by requiring registration.

To remove Pinterest from the image SERPs, simply use this when searching:

-site:pinterest.com

That may become Pinterest's downfall: Having to register just to view.

I can see why they are doing this. If more people register simply because they want to view the pictures then Pinterest can show potential investors a higher amount of registrations and "users".