Apple is creating a large marketplace where it controls everything. It can be argued that the 30% tax is probably reducing the amount of useful software being produced, as is the case with other store rules like browser engine restrictions. Also, it allows Apple to compete in unfair ways against e.g. Spotify has to pay the 30% tax if they want to offer the same service as Apple, which is to offer in app subscription options, where Apple pays nothing. It may or may not meet the legal definition of a trust, but it surely seems to have all of its negative effects.