Follow the money brick road...
It appears that Pinterest may have worked itself into becoming an affiliate site for iStock Photo.
Please be aware that the Pinterest website is a member of our Affiliate program. The iStockphoto team which handles the affiliate program, continually monitors the usage to ensure it is in line with the program.The use of the word "affiliate" comes with the expectation that Pinterest would receive commissions for licensing revenue that is generated by traffic referred from its pages. In other words, if someone follows the link to an image posted on Pinterest and purchases rights to use this image from, in this case, iStock photo, Pinterest would receive a commission.
It's not clear why anyone would want to purchase rights to a photo that's all over Pinterest, that's already on a pinner's own pinboard, or that any webmaster can hotlink to from the Pinterest servers without worrying about copyright infringement (Pinterest has the "rights" to distribute the image according to its ToS).
Not much noise has been made to make the photographers on iStock photo aware of this affiliate scheme - possibly because this news might upset some of them.
The money pie isn't infinite, ultimately, who pays for Pinterest's commission? To whom is the burden transferred? Now, and over years?
Is such an affiliate relationship a reason why a photo licensing site would be ignoring copyright violations?
How much extra revenue could this possibly generate? One might suspect "close to zero," but what are the real numbers? How many pinners care about copyright enough to license a photograph? And for what? Their blogs? Their fashion magazine empire? The travel books they are writing? Someone looking to license a photograph won't be looking for it on Pinterest when stock photo websites have convenient search functions.