Interesting that tethering is part of the standard plan on Telstra in Australia. What's the reasoning behind the restriction on some carriers? As long as I stay within a data cap what does it matter whether it is done on the phone or tethered?
"We can make more money this way" covers pretty much everything when it comes to telcos.
You can be irritated that this nickel-and-dimes you, but what you're dealing with is an allocation decision, not an evil effort to defraud you. AT&T is about as up front about the tethering issue as they reasonably can be; "tethering" is a product option every data customer is presented with when they sign up.
So understandably, there's no way my telco will allow me to tether with that plan.