You're right, sorry for not being more clear. What I intended to say was that Apple won't allow you to tether without approval on any networks. At least it defaults to allowing on all networks by T-Mobile.

Apple even respects T-Mobile's wishes on the matter.

I put a SIM connected to one of T-Mobile's $30/5GB prepaid accounts in my iPhone 5. When I try to turn the tethering function on, I get a prompt telling me to go to a T-Mobile website to get on a plan that supports tethering.

Ignoring the wishes of the people who gave them the better part of a thousand dollars for the piece of hardware in their hand.

The carrier control of tethering made sense back before the iPhone had market dominance. Now it's stupid and user-hostile.

That's just not even remotely true. What Apple does (to my annoyance) is allow the carriers to control whether or not the phone allows tethering. But they do allow it if the carrier does, and plenty of carriers allow it (usually after making you pay extra). I've been tethering with my iPhone on AT&T, officially, for years now.