Apple exerts more control over cellular networks than any other technology company on the planet. If a carrier wants to sell the iPhone they have to pay Apple a heft subsidy. Before getting the iPhone 5 to run on their LTE network, Apple has to come around and make sure it doesn’t suck before they enable LTE on the iPhone or iPad.
The reason behind Apple testing carrier’s networks before enabling 4G is pretty simple – Apple doesn’t want it’s users to have a crappy experience. But no other smartphone maker in the world has a policy where they get to test a carrier’s network before turning a feature on, it’s usually the other way around, and some carriers think Apple is getting a little too big for its boots.