Selling an iPhone app only through an iPhone store has zero to do with insuring it is secure on an iPhone. That is done in the development of the app, which can be checked with Apple before the app is released, and Apple software on the iPhone can verify an app being downloaded was approved or not.
The ONLY purpose for requiring an app to ONLY be sold in an Apple store or from their on-line store, is monopoly practices of Apple.
False. If you want, you can write your own OS to run the iPhone and load your own custom apps. Good luck with that. Horror stories everywhere regarding Android phones and apps
Android: "The application ... (process com.android.mms) has stopped unexpectedly. Please try again."
The reason for requiring an app to be sold in Apple's app store is to prevent problems like those rife with other non-Apple products.