At Apple they think they know better. Probably Steve Jobs himself is sure to know better than his clients and most people in this business. And he has some remarkable stories to prove this point, starting back in 1984 with the very first Macintosh which, coming only a few years after "buying a personal computer" meant getting home with a plastic bag full of electronic components, was closed with special screws allowing only professional support staff to open it. And even if you wanted to write software on it, you had to stick with some strict UI design rules which were unheard of back then.
But Apple is about user experience, it's about getting people excited from the moment they hold in their hands a cardboard box with a logo printed onto it. It's not about choosing, it's not about running your own software on an iPod, it's about smoothness, cool materials and funky effects when windows collapse, and they think they need full control of the whole environment to be able to guarantee this level of consistency.
I don't think that Apple's will be the only software running on iPhones (my guess is that there will be some kind of widgets architecture, probably compatible with the one running on Macs, and most probably developers will have to go trough the iTunes store to distribute their widgets), but nevertheless if you want freedom of choice go with Palm, Microsoft, Nokia or any other vendor: Apple will not allow you to control their toy. But this doesn't mean that they won't be successful. It looks like their strategy today could be described as:
Ask not what Apple can do for you. Buy our products and STFU.
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