Originally Posted by Kali Yuga
Why, because you declare it to be so? Are you privy to some contractual details we don't know?
Oh, and here apparently is the answer: They don't have the digital rights to some of the illustrations.
It's possible that if you re-download some of those titles, they've sorted out some of the rights.
It is a cheapskate thing because they don't want to negotiate for the rights. They'd prefer not to pay and use a generic cover.