This is a "have your cake and eat it too" problem.

IMO you can either be a dumb marketplace with common sense moderating and not be responsible for the content on your marketplace, or you can be a curated and secure marketplace, in which case you must necessarily be responsible for the content on it.

This is a legal hole.

1. Not endorsement, but at least a recognition of some sort that Apple recognized the dev and the app and allowed them to publish this app (regardless of which store).

2. AFAIK Apple isn't doing anything illegal by pulling out this app. Malicious compliance? Perhaps. Illegal? Nope. If Apple doesn't do this, then it would indeed attract legal issues due to the first point.