Even a cursory reading of Economics 101 will reveal that 'closed' systems, exclusivity and delivery pipes are what reliably generate revenue. So from AAPL's POV this is perfectly reasonable, as shown by the selling of 100+ million iPods and 3 billion songs through the closed iPod/iTunes/iTS triad.
I explore why those who blindly advocate 'opening up' right now may not be looking at a full deck of facts here:
iPod touch nay-sayers: shackled by “gadget thinking”
http://counternotions.com/2007/10/01/ipod-touch-nay-sayers-shackled-by-gadget-thinking/
Why The Zune Will Outdo The iPod