Who said anything about Apple having a monopoly in the mobile devices market?
Comparing Microsoft's monopoly on the desktop market to Apple's (lack of) monopoly in the entire mobile devices market doesn't seem valid because the scales are wrong. A better comparison would be Microsotf's desktop monopoly vs Apple in the phone market specifically (which is still not a monopoly and hopefully won't become one).
Also, surely if you look at the "app store" market (not THEIR app store but the whole lot of app stores that exist) rather than mobile devices then they do have a monopoly there? Then there is iTunes, which I expect will be the biggest digital music buying product (unless Amazon is bigger nowadays).