But now, Apple doesn't have as much leverage. If they walk away from negotiations and say "Sorry, no more AT&T iPhones," they're losing all of the customers that stay on AT&T and buy Android devices. And AT&T loses all of the customers that switch to other networks so they can get new iPhones.
Apple did try to do something about exploitative text message pricing when they introduced iMessage. AT&T's response was to stop offering anything but unlimited texting plans.