But the reality of the matter is that it's effectively impossible for them to stop? How do they attempt to stop unlocked phones?
No, it isn't impossible for them to stop. All of your traffic is going through their network and short of using a VPN to a remote system (or similar), there is nothing you can do to completely hide your internet traffic from them.
For the carriers, it is a question of how much scanning, blocking and filtering that they want to do. Too much and they get angry customers calling them en masse and going to a competitor. Too little and people exploit loopholes in the system.
T-mobile has been slowly (but steadily) ramping up their efforts to eliminate tethering. I actually went through this last week. I was on a very old (like 8 year old) data plan that did not mention tethering at all (meaning that it wasn't prohibited in the terms). It took them a few months, but when my data usage started getting close to my 5GB pre-throttle cap, then they started redirecting my computer even though my phone still worked just fine.
A lot of people have the wrong idea about tethering because they believe the crap that the carriers tell them.
Tethering is not a 'service' that the carriers offer. Tethering is something that your phone does with the data connection that you rent from your carrier. If you own your phone like I do, then it is your device using a feature that you enable to distribute data (which you already pay for) to your other devices. There is no extra 'service' by the carrier involved, despite the spin that they try to give you.
If a customer service representative tries to feed you some talking point about how its the carriers' data, then interrupt them and tell them that you want to speak to a level 2 tech who isn't chained to some script.
