The control that they over the iOS platform has provided the first truly safe computing environment. This is, in my opinion, more important to its long-term success than the innovative user interface. This is the part that other companies have so-far failed to copy.
Apple don't seek control for it's own sake—it's a lot of work and very expensive—which is why other companies balk at it.
To line their own coffers, of course. By banning apps that the majority of people don't mind (see: Google Voice unbanned), they're able to increase their own profits.
This is much like the difference between the US and PRC governments. The latter is able to optimize its economy to a much greater degree than the former, thanks to its top-down control. But at what price?
What line? Why else would they do it?
The control that they over the iOS platform has provided the first truly safe computing environment. This is, in my opinion, more important to its long-term success than the innovative user interface. This is the part that other companies have so-far failed to copy.
Apple don't seek control for it's own sake—it's a lot of work and very expensive—which is why other companies balk at it.