It's a convenience issue.
From what I understand about this Apple Sim, it's designed so that users can freely switch between carriers on their iPads without having to go get sim cards from those carrier stores and keep swapping them. Also it can be switched back and forth among carriers whereas if you end service with a carrier sim card, the card is deactivated and you need to get another one if you want to switch back.
Looks like with the Apple Sim a user could for example subscribe to T-Mobile when he's just around in his home city but purchase a one week plan if he happens to go vacation in a rural area for a week or something. Then just switch back to T-Mobile when he returns home. All this done right there from the iPad without going to different carrier stores buying several sim cards back and forth.
So what AT&T is doing seemingly just defeats the whole purpose of the Apple Sim. They're likely afraid of possibly losing tablet subscribers to other carriers. Since tablets are the largest source of subscriber growth for the US carriers these days.