They seem to have a monopoly on marketplace hype, I won't doubt that. But to claim they've a monopoly on smartphones is just plain wrong.
The Android app store certainly exists, and is doing quite well. The web is free for all platforms. Apple don't, by any realistic measure, have a monopoly on application sales for smartphones. They just couldn't - with only 25% of the smartphone market, and an app store that targets only that 25%, how can they have a monopoly on smartphone app publication? What they might have is a monopoly on the wealthy gadget consumer with large amounts of disposable income to spend on apps. I'm not, however, that Apple's clever demographic targeting, coupled with the inability of their competitors to eat into this market via fair competition, is reason to force them to change their business model.
So I don't think we can say "they just couldn't" — in fact, they just did.
And I might be skeptical about the numbers (Gartner isn't infallible), but every story I've heard from cross-platform mobile dev houses agrees with this — you'll get almost 100 times the sales on iPhone that you will on other platforms. I've never heard a contradictory story either, not even from Google trying to puff itself up. That's pretty much a monopoly.
Regardless, one does not need a monopoly to engage in anti-competitive actions and for them to be illegal.
So yeah, I think an inquiry is an appropriate course of action here, for the same reasons it was in the 90's with microsoft. If you "tend to agree with antitrust action most of the time", but not this time, I think you might be better served by introspection than posting a defense of Apple.