Not all that objectionable IMHO, even if you think Apple should allow other app sources. Of course an app store having clear policies and a good reputation is an important factor for users installing apps from there - and thus for vendors picking one to sell in. You see that in action on Android to some degree.

EDIT: In some ways the best option IMHO would be a package-manager like design, where the system is responsible for fetching and updating packages, verifying signatures, security prompts, ... and "app stores" just feed information into that, but I doubt we'll see that anywhere.