I thought the whole point of the "Apple SIM" was being able to switch carriers any time you want?
If that doesn't work, then why did they even bother? Might as well just use official carrier SIMs.[/QUOTE]
While it would've been nice for the carriers to play ball a bit better, the fact remains that they still want control over how devices get activated on their network. AT&T does this by putting silly restrictions on how SIMs get used. Verizon does it by refusing to use anything but their own SIMs. Sprint does it by locking down their network and only allowing whitelisted devices.
Ultimately though, the true reason for the Apple SIM is fewer inventory headaches for Apple. They don't have to have to have multiple SKUs and stock multiple, carrier-specific iPads which have a special branded SIM for each carriers pre-installed, when they have a single SIM that works on most of the US carriers. So you no longer have a situation where an Apple store is "out of" AT&T iPads but has a bunch of, say, Sprint iPads. Now it doesn't matter, they hand you whatever iPad off the shelf and what happens after you do your first activation isn't Apple's problem anymore.