Since apple have a 'review process', and the app store isn't a 'stuff gets put up, until it's taken down' deal, but a situation where apps have to be approved by apple's reviewers. Apple may well be legally liable for any infringing work that is sold on the app store, so taking a 'get a court order and take it up with the developer' MAY leave apple open to some liability of damages, whereas just yanking the app at the first sign of trouble will probably mitigate some of that.