Crediting the source doesn't mean it automatically falls under fair use.

Pinterest's ToS state that you cannot pin material if you don't have copyright permission. Of course, this is probably a large percentage of what gets pinned.

You're not allowed to redistribute copyrighted material without a license. That is the entire point of copyright. And most of the content of the site is directly infringing.

The "almost always" part is a bit of a problem. Why does Pinterest allow people to remove attribution? If Pinterest cares about respecting copyright, the simplest step would be to preserve attribution (and on a practical note, the resulting referer logs would notify image creators that their stuff is being pinned).