Last I checked, Apple had something like a 20% share of the market for the platforms on which those apps run. Their phone has a feature, which all other app phones share, that happens to get used far more than its competition. That doesn't make Apple a monopolist.
They sell near 100% of all apps for all phones. The people who buy apps on Android, Blackberry, Simbian and WinMo are vastly dwarfed on the sales of apps for the Apple AppStore.
They do not have a monopoly in phones, but in App sales.
Explain to me how you can have a monopoly in app sales when you own only 20% of the app-running platforms (and you have competitors with a significantly larger share). How can you distort the market from that vantage point? If you abuse customers, they simply go to BlackBerry or Android.
Most people who own android, blackberry and winmo, don't buy any apps. Most people who have the iPhone, buy 27 (the actual number, the mean as of April 25, 2009).
That's how. A huge percentage of android apps are free, and many people don't get you can install things on blackberry and winmo.