Generally, people in the U.S. buy a phone from the telco itself, and specific companies customize the software and packaging of their phones. Sometimes, different models are produced for sale by specific telcos.

I understand and agree with the outrage you feel, but the status quo is powerful and slow to change. There are telcos that explicitly provide tethering in their plans as a standard feature, and bringing an outside device to operate with the telco is easier than it used to be, so maybe things are starting to shift.