As to those who are wondering why they have to pay extra for tethering, are you really surprised that a company would charge more for a service that you value, which will result in heavier usage of their resources?
If it results in heavier use, they should just charge for the use directly, not micromanage how I use the service. In the freemium scenario, it would be like charging extra to be able to access the website from more than one computer. Both are stupid because both require nothing to "support"; they inherently work as part of the existing infrastructure. On the other hand it does take resources to try to block that functionality, because you have to actively look for "cheaters".