I've written before about AT&T's capped data plans and what I feel is a lame $20/month additional charge for tethering. Since you are already paying for the amount of data you use, why should you have to pay extra for tethering? I suspect there is some marketeer at AT&T who recognizes there's a strong demand for tethering so they can charge extra, why leave money on the table?
What the wireless carriers are missing is that demand for wireless services is a huge growth market. By increasing capacity and driving down prices, they could dramatically increase the size of their business.
The PC revolution is going mobile. We need 10 or 100 times the bandwidth currently available. If you build it and price it right, you'll attract way more business than by trying to monopolize on the current shortage of supply.