For Pinterest, Revenue Would Turn Copyright Questions Into Problems

To be honest, I'm a bit confused about this issue: How is this different from a user sharing copyrighted images (ie: memes) on Facebook or Twitter? Or even just plain ol' sites dedicated to memes?

Are they getting targeted because of their popularity? But what about FB? Is it because Pinterest is open doors where FB is (somewhat) closed?

Would Pinterest avoid all these issues by simply linking the images instead of storing it?