There are so many of these threads here and elsewhere. The way I see it, as an iPhone user on an iPhone contract, Apple developed these phones, and have advertised them as only being available under certain contracts on certain networks, worldwide. There are various reasons for this, ranging from the fact that the iPhone uses data on the network automatically, to seemless syncing with iTunes. All I can say, it that in my experience it works utterly seemlessly.
Each time I read about iPhone problems, it is because people have unlocked the phones, hacked the phones, put them on other tariffs and then seem annoyed because the phone, for example, connects to the mobile data network, incurring them large charges. When I read such stories, I just think, well if you hadn't unlocked the phone, you wouldn't have such difficulties ...