I believe t-mobile forced that on apple. T-mobile sells enough Apple phones that they can call Apple and say we need this feature in all phones we sell and Apple will do it. It isn't clear who would lose more if t-mobile decided not to sell the iphone but it isn't one Apple would want to risk.
I have no doubt Apple already have people wanting to do this which is another reason Apple wouldn't push back. They already had people wanting to do this, and so getting a few business people who otherwise didn't care on-board is enough to push the decision.