Naiveté and Myopia Abound.
Part of the deal with the phone carriers is that Apple would be the face of the store - so it is up to them to do the banning, but if you think they didn't have a push in that decision then you are shockingly naive in matters of business partnership and primary corporate goals.
Apple's primary goal is to sell hardware, and every action they do is to promote that goal.
The Wireless Company's primary goal is to sell bits as they travel back and forth between devices and radio towers. Everything they do is to promote that goal.
With this in mind, every decision that has ever come out about the iPhone makes perfect sense.
(Many things still suck - but they make sense)
It's amazing to me that not only the OP, but every commenter holds the belief that Apple cares how much bandwidth you use on the cellular network.
Apple makes it's money selling you physical tangible objects. Restricting what you can do with that object is only done at the behest of partners. The RIAA imposes DRM in the iTunes stores, and the Wireless Companies impose bandwidth restrictions.
The only thing that Apple digs it's claws into is the OS, and that's because they believe the OS belongs to the hardware - not to the end user. It's Mac OS X, not End User OS X. (That's why software is licensed but hardware is sold.) ...and doing so promotes the primary goal of selling hardware.
Apple developed an OS to sell hardware, and they lock it down for the same reason.
How much bandwidth you use on a cell network doesn't affect Apple's goal either way.
Guess who's goal it DOES affect?