alt Hacker NewsApple isn't the most dominant business on earth, so maybe they can't do anything in violation of antitrust?
Not how it works. You define a relevant market, and the market of iPhone users (which a stable market, there are barriers to just switching to android tomorrow) can be used for evaluation.
This really is a grey area. You can't define "the market for electricians in my local town" and get any antitrust protections. Defining the market is a huge part of how politicians make this political and therefore something they can profit from.