This has always puzzled me. Why would they NOT do it? Apple does not control smartphone market - they control iPhone market, a subset of smartphone market. They have made the product, the supporting infrastructure for it, how are they obliged to allow competition in there?
For example, some time ago I bought a Canon printer. There was a booklet packed with it, telling me what inks can be used with it - all were of Canon brand. Now, I know there are some third parties selling inks for Canon printers, and without a doubt they would like to be featured in those booklets, preferably with the notice that their ink is of comparable quality while costing far less. But Canon is not including them - is that a dick move too?
Yes, you can happily use X ink in your Canon printer. And you can run X adds in your iPhone apps. But those inks would not be sold in Canon stores. And the same way, Apple can choose not to distribute those apps in their Appstore.
Last time I checked, neither iPhone platform, nor Appstore were public domain :)