Yes, you are right. But the point is that it is impossible for a third party to make a music playing App that plays music in a users' own iTunes music library a

Yes, you are right. But the point is that it is impossible for a third party to make a music playing App that plays music in a users' own iTunes music library and that has all the functionality of the corresponding Apple app. Note that I just chose this example because I am familiar with it. There are many "private" frameworks, all of which (a) offer functionality and (b) are off limits to non-Apple apps.