The reason AT&T charges you to tether probably has little to do with technology and everything to do with segmentation. They have two choices. They can raise their data rates for everybody and attrit customers to other providers (or to less lucrative featurephone packages). Or, they can find ways to charge higher rates to less price-sensitive customers. You are a person who wants to plug their computer into their phone and use it to get on the Internet from the Acela train. There are not that many of you, and you are uniformly wealthier than most other segments of AT&T customers. Finding a way to charge you for that behavior is marketing 101.
If you're an entrepreneur, it's good that the rest of the world sees things AT&T's way (to wit: when you agree in a contract that you're going to use a service in way X and not in way Y, the provider is within its rights to enforce that contract). It's this force that allows you to charge money for things and thereby make a living.
If you're anyone earning the anomalously high income accorded to technology professionals, you're run a slight risk of hypocrisy complaining about this. A complex web of similarly nerd-arbitrary contracts keeps the money flowing through the conduits and manifolds that eventually feed your bank account.