...in the United States, you can choose a domestic plan from either Sprint or T-Mobile and also pick an alternate plan from the other carrier as needed. When you choose AT&T on iPad Air 2 and iPad mini 3, AT&T dedicates Apple SIM to their network only.
AT&T: "Apple wants to do what?! Make SIM cards simpler?! Nah, we're good."
I almost don't want to know what Verizon would have done.