When you share content from gimmebar, they're no longer sure the original source is still displaying the item in question, or even that the URI chosen was canonical (think the root url on a blog). What choice do they have?

To go a step further, I think you're freeing the person sharing the content prematurely. If I see something funny on cracked.com, and decide to grab that content, upload it to imagur, then share the imagur link… Imagur getting the traffic instead of cracked is on me, not imagur.