If I publish a review of a work of art, including a reproduction of the work, in a magazine, my copyright would cover the entire article including the reproduction. I wouldn't be claiming copyright on the original work.
I too find this insane. If anything qualifies for fair use, surely pinterest does. "Hey look at this cool thing!" is about as close as I can imagine to the platonic ideal of discussing a copyrighted work.
No one freaked out over /r/pics, so what's the deal here? I hesitate to point out that pinterest differs mostly in the gender demographics of the user base, but... yeah.
/r/pics is just a collection of links to images; it does not reproduce or redistribute the images.
It has nothing to do with whether or not /r/pics constitutes fair use, just if-it-isn't-fair-use-who-gets-sued? No one, at the time or now, seriously thought that there was a legal problem for anyone with reddit. So why pinterest? Again, part of me is really suspicious that it's because it's a chick site that doesn't cater to geeks.