I never looked at Pinterest or other sites like that, hell even Facebook and other social media sites do the same thing. When posting a photo on Facebook it doesn't ask if you are the sole owner of the photo but you can upload just about any none mature related image.
But I don't see this as a "huge" copyright issue, people aren't taken credit, selling, or doing whatever to the photo, just pinning it for whatever reason. If the photographer doesn't want this to happen then don't put it on the web or watermark it.
I know when I put stuff on the internet people might use it and that is fine as long as they don't take credit for making it. I use to design Forum Signature art and I come across people actually saying they made my art and even one person who was an impostor with my name and artwork on another graphic forum. So I feel I have a good understanding of both sides. But I look at the web as common use when it comes to Social media outlets.
Final point, when people ask where to get stock photos from I never say google because you can't contact the owner for permission to use and I do believe it is copyright infringement, I always recommend sxc.hu which you can get permission and use for whatever your purposes are.