I know this is an old topic, but I've been pretty fired up about this lately. I understand that you think this is the reason why they do this, but I'm a little skeptical. It seems to me that people who are barely using their data plan probably wouldn't even notice that tethering is a feature they could use, or would want to use. I really don't think it will have as big of an effect on the average data consumption as you, or AT&T, would think.
With that said, I e-mailed customer service when the data plans were first announced, asking why they thought they were justified to charge for tethering when you're already paying per byte. This was their response:
"Historically, all of our Smartphone Data plans have required an extra charge for tethering, as do other providers. This practice is continuing with the new plans, so that we can stay competitive with the other providers plans. Again, I do apologize for any frustration that we charge extra for tethering."
Basically, "other carriers do it, we've done it before, we're going to keep doing it". I think that's a pretty bologna answer, I would've been much happier with an answer like the poster above gave, but I don't think AT&T trains their customer service responders that well. On top of the fact that if they lifted the charge for the feature, it would help them stay competitive MORE, because they're offering something their competitors don't offer!
And even if what the above poster said is true, that by including tethering, the average data usage would go up and they'd have to charge more for data plans, that's fine. Charge me $5/month. Maybe even $10. But to nearly double the cost of your data plan just to use that data on another device is disrespectful and insulting. Would the cost offset really be that much? If they put the tethering plan at an additional $5/month, would the average data usage go up that much more? I understand that they probably did a lot of research to show that $20 might be the best price point to mitigate the usage, but they aren't even saying that. Instead they're saying "it's an added value", even though it's not a value that THEY add!
All in all, I'm pretty annoyed. I'm planning on started a complaint website, similar to fixoutlook.org, where users can have their voices heard and maybe even send harassing e-mails to the CEO (whom is now a much nicer one, btw). I'm especially annoyed because I want to get the new iPad and I believe I'm entitled to use the 2Gb I already paid for on that device (go ahead, make fun of my American sense of entitlement, I think when I pay for something it should be mine). I hope the OP sees this post and is interested in getting in on the action for the website. Maybe engadget and gizmodo will promote it and we can actually get some momentum to use our responsibility as consumers to make change. The only reason captialism fails is because the consumers don't take this responsibility seriously, and just let companies call the shots. Let's not be afraid to make waves!