Apple is manufacturing and selling telecommunications devices. That being the case, the FCC's purview extends well beyond simply clearing the devices for interference. That's the standard a device is held to if it may emit EM radiation, but that emission is not meant for communications purposes, AIUI. The bar is set rather higher, and the FCC's powers rather deeper, if the EM spectrum the devices emit on is actually meant to be used for communicative purposes -- particularly if it uses a licensed band.

Disclaimer: IANAFCCW (I am not an FCC wonk.)

Is this a fair shortening of your comment:

Because Apple sells devices that use the public airwaves, they are subject to regulation, and the agency in charge of that is the FCC?

There are no legal proceedings, the FCC is just asking so it can have the full story. If AT&T is leaning on Apple, then it is possible that AT&T is illegally using the radio spectrum we licensed to them. They need to know what's going on, though; they don't want to guess.

You've hit the nail on the head. It's not what Apple's doing - it's what the carriers may be doing. Given the way the carrier market works at the moment, consumers do deserve competition protection. The FCC is quite right to stick its oar in here, even it it's only to say "hey guys, I'm watching you".