You must be new around here. ;-)

Microsoft used their natural monopoly in operating systems (Windows has 95% marketshare) to force OEMs (HP, Compaq, etc.) to bundle IE with their machines and not Netscape Navigator. Microsoft threaten to cancel their Windows licenses.

And later, it decided, against its decent decree with the US government, to bundle IE with Windows, claiming Windows wouldn't function without IE, which was a lie.

Perhaps you're not aware that natural monopolies are themselves not illegal; it's using that monopoly to force the market to do things it ordinarily wouldn't in some other area.

So… no, there are no antitrust issues regarding web engines on iOS. And of course, Google, Mozilla and others have browsers on the App Store that use WebKit anyway, so it would be hard to make the case they're being harmed…