Because without a dominant position in the market you can't effectively stifle competition.
But they are stiffing competition. Competition building apps that run on their hand held devices. It really can't matter that the devices aren't dominating the market, because the market isn't smart phones apps, the market is Apple hand held device apps.
They are a closed system so if I want to make apps for Apple hand held devices, which apple holds a 100% market share of, then I can't unless I jump through their hoops and don't compete with their apps like Safari.
This is the very definition of stifling competition.
How can one look at this announcement and think otherwise?
Blackberries don't run iPhone apps so it doesn't matter how many alternatives there are to iPhone if a developer wants to enter the iPhone/iTouch/iPad/ app market they are stifled.