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.
i guess they could put a hoverbar at the top of the page when they link to the original content. if there's a 404 or redirect, the user can click the hoverbar to see gimmebar's copy.
i'm not sure how to do it, exactly, but i know it's possible to do this in a way that isn't so... swipey.
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.