The embedded sim is not marketed as "flexible". It's just that - an embedded sim. Use it. Or not. But don't accuse AT&T of doing something they are not. You can use whatever sim you want. The iPad in unlocked.
Well, it IS marketed as flexible. It's the whole point of the Apple SIM card... flexibility.
http://www.apple.com/ipad/apple-sim/
"With Apple SIM in select iPad Wi-Fi + Cellular models, you get the convenience of choosing a cellular data plan from carrier partners right on your iPad.1 And when you travel,
Apple SIM gives you the added flexibility to stay connected in more than 90 countries and territories around the world. So when you reach your destination and want to check your email, find directions or send a message back home, you can purchase a cellular data plan for the duration of your trip — whether it’s a day, a week or a month."
"Apple SIM lets you keep your iPad connected in more than 90 countries and territories around the world. When you reach your destination, simply select a cellular data plan right from your iPad and get connected. Since iPad is an unlocked device, you can also install a local carrier SIM if that’s a better solution.
And because Apple SIM is so flexible, when you return home you can easily switch back to your U.S. carrier plan."
[doublepost=1459605318][/doublepost]If this article is correct, it sounds like AT&T will only lock the embedded Apple SIM if the iPad is purchased at one of it's stores. Fair enough, but buyer beware!
http://recode.net/2016/03/22/latest-ipad-pro-makes-it-even-easier-to-switch-wireless-carriers/
"T-Mobile and Sprint are fully supporting the built-in Apple SIM feature.
AT&T, however, will tie the Apple SIM to its network if you buy your iPad at one of its retail stores. Verizon, meanwhile, will require a separate SIM card and disable the built-in embedded Apple SIM on the iPads it sells."