I agree that unlimited users should have to pay to tether for the reason you stated (but not $20.) But If you have the 2 GB data plan, you still have to pay AT&T for the privilege of tethering. You are using the exact same bandwidth, just in a different place. You shouldn't have to give them money to use bandwidth that you've already paid for.
Not true. Full computers chew through more data than a phone. When you stream a youtube video on the computer, you can choose high def (as high as the computer will handle), but it would be over a 3G connection. Therefore, you're loading a lot more data than the iPhone would with youtube.
Computers use more bandwidth than phones. You should have to pay for that extra bandwidth use. Also, computers can load pages faster and have longer battery life... therefore, you'd be hitting up a lot more webpages for perhaps an even longer time than you would on a phone.
The way I browse the internet on a full computer is nothing like the way I do it on my iPhone.