You can't escape the fact that when you sign up for cellular service from a provider you sign a contract that contains certain terms and conditions... enabling a wifi hotspot on devices that don't include that service as a part of their plan, or an added feature to their plan is contrary to the contract and terms & conditions.
Sure, but as I said, when Unlimited Data customers first signed up, the iPhone was exclusive to AT&T. Tethering wasn't an option & we were all told that the unlimited data would truly be unlimited "without" any restrictions (which it was). This policy was fine & dandy as it persuaded millions of its customers to upgrade/update their contracts... we were all told yes, you'll keep the exact "same" unlimited data when you update when in reality this was obviously a rouse and not the case.
The fact is, AT&T (& Apple for that matter) realized that as mobile technology started trending & customers started consuming much more data, subsequently their networks began taking a hit. A poor public perception of AT&T service would also project negatively on Apple as a whole. Their solutions, reduce your usage by limiting some apps from functioning on 3G and eventually by throttling. Do you not remember the outrage amongst unlimited data users when AT&T first started implementing the throttling?
Of course you can argue, well you agreed to the fine print but I will also argue that AT&T's practice of pushing customers to agree to new 2 year contracts by "reassuring" it's customers that their plans will be exactly the same is BS. AT&T & Apple's original exclusivity deal helped them both prosper immensely. Back then, if you wanted an iPhone, it was AT&T or nothing.
BTW, I'm not saying my opinion regarding tethering is "currently" justifiable but neither was AT&T's bait and switch contract upgrades. Everything isn't so cut & dry. I was just stating that the comment made earlier is NOT necessarily true... "AT&T will catch & change your plan".