It's just a choice Steemit is making as far as how and what links display on their platform. It's up to the platform to decide what will display from a link. For example, when I post Steemit links on the Minds.com platform, the source image shows up on Minds.com automatically, so basically Steemit is trying to make sure that it's better for me if I post on Steemit and share on Minds.com rather than the other way around which will leave me with a text-only share unless I put up my own image on Steemit, which might seem like no big deal, but if it's not my post then I might not have the right to post the image myself and probably won't know for sure whether I do or not.