Whether or not it is a monopoly is a red herring, since being a monopoly is not illegal. Abusing a dominant position in a marketplace is. Apple has dominance

Whether or not it is a monopoly is a red herring, since being a monopoly is not illegal. Abusing a dominant position in a marketplace is. Apple has dominance in the Apple app marketplace, which they could theoretically abuse by discriminating against competitors. Thus, it is in their interest to be transparent about how they're managing their app store.