On the iPad they have the ability to create virtual SIM's and you can select from a list of providers, including one-time payments to activate an account for X amount of data.
Apple could move to something like Google's Fi easily with the virtual sim capabilities...
Apple doesn't need AT&T anymore - the original exclusivity was because Apple didn't want any carrier interference (as carriers love to add their logos to handsets, install crapware into the firmware, lock-down settings and features, etc) - none of the carriers would agree except for AT&T, who demanded an exclusivity period to compensate for the risk of loss-of-revenue from evil things like only allowing photographs to be sent by MMS.
Apple could move to something like Google's Fi easily with the virtual sim capabilities...