The reality here, however, is that if Apple wants to protect themselves from liability they have to follow the rules in the DMCA. That means they have to take down reported infringing content. It can be put back up, if they receive a counter claim. However, there is a minimum period that it has to stay down. I don't have in front of me, but it's a few days.
The reality is that Apple does not have to take anything down if the artwork in question is not contained in the package Apple is distributing, and the DMCA claim has to include the reference to the asset in question, making it easy to see if it's distributed by Apple or not. If a third-party server the application can optionally connect to is violating the DMCA by distributing content illegally, that's legally not Apple's problem (in the sense it's not Google's fault that Google Chrome can be used to visit The Pirate Bay) and Apple could tell the claimant to send the claim to the appropriate place instead.
However, AAPL as a corporation has a large financial incentive to ensure people pay for things they could otherwise get for free, so they'll do everything they can to scratch the backs of other arsehole technology companies while screwing over alternative applications which attempt to undercut them. We know this because they also discriminate against generic, powerful software on a whim for similar reasons even when there's no DMCA infringement claims to hide behind. Gnutella, BitTorrent, IPFS and many other P2P protocols are almost always disallowed, forcing developers to hide P2P clients inside of other generic tools in ways Apple's reviewers can't easily detect or ban.
Every time I hear a story like this, it reminds me to keep at least some FOSS-friendly hardware available and well-maintained, as we never know when we'll otherwise get screwed over.
We are drowning in information but starved for knowledge. -- John Naisbitt, Megatrends