Nothing to see here
The US antitrust authorities went after a Microsoft that had roughly 95% of the entire computer market, and had clearly exercised that monopoly to crush other businesses and prevent any effective competition.
Apple is tiny by comparison. I don't think anyone has a handle on its market share, but let's say 10% of the laptop/desktop market, and 20% of smartphones/mobile devices. Apple makes no effort to obstruct or crush competitors, and many are thriving.
Apple is targeting a premium market, and many other businesses want a slice of that pie. But if you want your business to have a future in that market, you have to consistently deliver quality. Despite all the bigoted protests that Apple customers are sheep buying shiny toys, Apple only retains a loyal customer base by delivering a premium experience. You won't stay in business charging premium prices whilst delivering rubbish.
Quality is hard to maintain, and it has to be reflected in the Mac App store too. And this is the whole point of the rules - your applications are expected to reach an excellent standard. Apple doesn't want to unleash 1000's of copy-cat fart apps, cheap knock-offs, malware, or crash-prone software on customers who have paid for quality. Steve Jobs is very succinct on that point - if that's what you want, there are other companies that will provide.
Apple has no ambition to rule the OS/computer/phone world. It has clearly delineated the market its targeting, and is perfectly happy for consumers to go to other businesses if they want something else. Antitrust bodies will have no problem with that - their job is to allow that exactly that kind of competition to exist. And Apple has plenty of competition.