Why not? They provide a lot of other policing work, like use of wrong APIs, style violations, rejecting your app because there are already enough apps of that kind and you added nothing new, and all kinds of things way more fiddly than wholesale copying. They're taking a nice cut, they ought to be bringing the customer and developers some value for it. Isn't that the whole Apple Store value proposition?
What Apple does is curate the store to their standards, which makes them the decision makers on what is and is not acceptable. Copyright is a large and nuanced legal issue in which they can't be the final arbiter, so why would they want to expose themselves to that hassle? This exact issue is a great example: according to the "copiers", they have a legal right to release the app. Whether or not they do is not up to Apple to decide.
The whole point of the DMCA takedown process (for better or for worse) is to remove the burden of investigating the validity of copyright claims from distribution services. It's one thing to judge app quality; it's another thing altogether when two people submit essentially the same app, and disagree over who actually created it.
Why not? They provide a lot of other policing work, like use of wrong APIs, style violations, rejecting your app because there are already enough apps of that kind and you added nothing new, and all kinds of things way more fiddly than wholesale copying. They're taking a nice cut, they ought to be bringing the customer and developers some value for it. Isn't that the whole Apple Store value proposition?