Old thread, but it just isn't true that Apple doesn't have the ability to determine this. Apple is choosing to not allow tethering apps. It is hard to imagine any justification beyond trying to appease carriers for favorable treatment.
Tethering is not just a carrier service any more than streaming video is just a carrier service. It is service that uses your data plan that carriers try to make money off of. In fact, tethering is a feature of a phone and its operating system, and all that 3rd party apps are used for is configuring the phone to perform a feature that is already available based on the hardware. Based on Verizon's 2012 settlement with the FCC, Verizon explicitly says that they allow third party tethering apps as it is our legal right to tether if we are on a metered plan.
Apple voluntarily chooses to not allow tethering apps in their app store. It may be that carriers promote iPhones more as long as Apple plays this game. This hurts users as they have a legal right to tether to a phone on a metered plan, but Verizon has no obligation to facilitate this by providing an app and Apple doesn't want to allow 3rd party apps. It is Apple's right to not help us tether, but it is a disturbing choice.
I have been considering purchasing an iPhone 6 Plus since I have games I bought for my iPad (XCOM, Baldur's Gate, Avernum) that would be fun to play on my phone, but it looks like Apple's not allowing the completely legal tethering apps means I would either have to jailbreak the phone to get such an app from Cydia or pay $20 a month to use the Verizon feature. Keep in mind that the $20 does not buy the right to tether -- you already have that if you are on a metered plan! It merely gives you a few more gigs of data per month, and enables the built-in Verizon dependent method of tethering through your phone. Read the agreement with the FCC from 2012.
I was also at a Verizon store since my upgrade this month will change my plan from unlimited data (with no right to tether) to a 6gb/month plan at the same price. They also said I could pay $20 more for 8gb/month + tethering. I replied that my understanding is that if I am using a metered plan, I have the right to tether even without paying the $20. The salesperson said, 'yes, you are allowed to tether with that plan'.
So if you have a metered plan on an iPhone and are unable to tether to your phone, there is only 1 party to blame -- Apple. I know there is a very manual workaround using shell scripting, but until Apple has a more customer focused policy on this, there is no guarantee that the manual workaround will continue to work long term if Apple deems that making carriers happy is the smarter move. I will probably follow up with the Apple store regarding whether the built in wireless tethering functionality depends upon the Verizon tethering service (that they provide along with extra data for $20/month), or whether it will work so long as my plan is eligible (any metered plan according to the FCC).
I do not blame Apple for being cutthroat. They know that they have to a great degree a captured market in those who have invested in a lot of apps and they know that most consumers don't realize that they have the right to tether, even without paying an extra $20. If playing nice with carriers means carriers promote their phones more, it seems a smart move. The only way that changes is if enough people complain. Unfortunately, people only realize there was a problem when the problem is gone, kind of like how many didn't realize that the iOS Google Maps was missing the turn-by-turn navigation instructions that were already in Android until they got turn by turn navigation.