It's a brilliant, (albeit arguably evil), strategy by Apple to create a viscous circle where developers bind themselves to iPhone, which means the good apps wind up there and thus users buy the product and pay for superior apps, which makes it more lucrative to develop for, and so on. And they sell some extra Macs and make money on dev fees, and take a cut out of every app store sale. It's really a simply ingenious grand strategy which has worked unbelievably well for them, however unethical or limiting some people might think it is.