What makes the connection between pinterest and flickr so interesting to me is that (if I remember correctly) A THIRD of pinterest images come from flickr, and pinterest thrives on using flickr's awesome repository of images against the law and the spirit under which most of the images were uploaded.
I believe I know why they are not hotlinking... if they did, they would immediately get a sh!!load of complaints from flickr authors about their images being used that way. (As anyone can attest who ever hotlinked flickr images on their sites, even WITH author attribution.)
If I were flickr, I would implement a feature "Your image has been pinned on pinterest!" with a notice that the views on pinterest will not register in the flickr stats, and allow the author to send a take-down notice to pinterest by pushing a button.