There might be reasons for this, which I'm not aware of. I'm posting this so we have a specific place to discuss it. I never understood why we say 'Coce cover image' or 'Amazon cover image'. Both the end user just want to see a cover image, and not have the screen cluttered with useless texts.
Some services might require a link or the mention of the source. I haven't checked for the ones in question, but I know it's the case for one of our German cover providers.
AFAIK also one of the larger providers requires you a backlink to their shop including their name. (That's why we don't enable their covers, btw.)