If you define your market small enough anything can be a monopoly. I bet Apple has a 95% market share on phones that cost more than $1500. I bet Google has 75% market share on phones less than $100. So what?
Apple's control of the app store makes the $1,000 phone more appealing. If they opened up the iPhone to other random app makers, the phone would be less appealing and they wouldn't sell as much.