Apple has a 100% monopoly on the market for iOS app distribution, which can be argued is bad for the customer because the 30% revenue share they take is inflating app prices.
People keep saying this but in an antitrust case you cannot simply declare the narrowest market that fits your argument and expect the court to accept it. The court will examine the market reality and examine how consumers actually behave to determine what the actual relevant market is, and it's unlikely that a court would find "iOS app distribution" to be a separate and relevant market for antitrust purposes. I've explained why in detail elsewhere [1], but in short, the US legal system generally does not consider an aftermarket consisting of a single brand's product to be a relevant product market unless specific rules are met:
> Because it would be inappropriate to punish a firm for its natural monopoly in its own products, courts embraced a sweeping prohibition against analyzing alleged anticompetitive activity by focusing on single-brand relevant markets: "[A]bsent exceptional market conditions, one brand in a market of competing brands cannot constitute a relevant product market." [2]
For a much more thorough explanation see my linked comment below.
