Most photographers who publish on flickr have a keen interest in their stats: how often their pictures have been viewed etc. With pinterest, the flickr stats do not reflect reality anymore. Thousands of users could enjoy and share the image on pinterest without the author ever knowing about it.
No image on the web is safe from copying, but I find it strange that pinterest makes a copy of the image and chooses to show it without clear, evident author attribution (including the "All Rights Reserved" notice, if applicable) when Flickr allows hotlinking and provides an easy api to pull the author and license information.