But Apple doesn't have a majority of market share (by number of devices), so, in my opinion, antitrust regulators have completely discounted their behavior.

On the other hand, the experience of using Apple Pay is miles beyond Google Checkout/Wallet/Pay/Android Pay/whatever marketing's new name-of-the-week-is, and only allowing One Blessed Way of taking payment on Apple devices is a big component of that. Having one consistent flow that I can execute in three seconds from muscle memory really does make buying-from-random-apps-and-retailers experience much nicer and more predictable than it is on the web and on other mobile platforms.