Pinterest is causing backlash because it is a theft tool, designed specifically to get people to "upload" images from other sites, and with a business model planned entirely on that.
I read in one article that they are currently in talks with PicScout (owned by Getty Images) about potentially getting their image fingerprinting software, so they can check user "uploads" against a database of fingerprinted images. But that will protect only the big boys, such as Getty. Plus potentially protect Pinterest from getting sued from those big-boy stock-image owners, if they check all images against that database.
It will not protect all the photographers, site-owners, and others out there who owns images. And there is no way that Flickr should suddenly have to register fingerprints with Pinterest for millions of images just to keep Pinterest under control.
Pinterest is going where Napster went before them. The law give owners copyright protection by default. Not just protection if you specifically ask Pinterest to stay away. Just like I do not have to post a sign on my house that says "burglars, please do not enter here". That is the default assumption.
Giving "credit" is not enough. I cannot legalize breaking into your house and stealing your stereo by putting a sticker on it that "gives you credit" for my new stereo.