This was more true in 2008-2010 maybe, but recently Apple has been much less restrictive. There are 3rd party browsers (chrome, opera, Firefox)*, 3rd party mail apps(gmail, Firefox), and 3rd part maps apps (tomtom).
That said, the lack of third party transit from apple is pretty horrible, and the API for the maps app is pretty limited (you have to throw the start and end points to your app, there's no overlays or anything like that)
That said, the lack of third party transit from apple is pretty horrible, and the API for the maps app is pretty limited (you have to throw the start and end points to your app, there's no overlays or anything like that)