I think this argument fails because if a person is tethering and using 1 GB an hour, they will burn through the 2 GB limit in two hours. After that, they will be paying extremely high overage rates, which will then deter them from continuing to use that high bandwidth. So, while they may be using much higher bandwidth for a very short period of time, having the 2 GB restriction will prevent that from being a problem.
I think the missing piece is that AT&T (and other providers) do not build their data network to support the maximum volume of data that they offer with their plans. Take me as a typical user who "pays for" 2GB of data right now (no tethering). I use 400-700MB a month -- somewhere around 20-35% of my "allotment." I don't think I've ever used more than 60% of what I paid for. I'm not atypical. If anything, there are many users who pay for data and use even less than I did.
Forget tethering for a moment. What I'm saying is that, if I and each of those other smartphone users went out of our way to actually USE 2GB of data each month that they're paying for, every month, just from their phones and not via tethering, the data network would perform terribly, because this is a lot more load than it's designed for. It can't handle that, whereas it can handle the average 400 or 500 or 600 MB that the 2GB users pay for.
What tethering would do is shift that average up substantially, without giving the provider any magical way to handle the additional bandwidth.
This is the same effect as when there's an emergency and everyone gets on their cell phone at the same time and many cannot successfully place a call. And the same thing would happen if everyone started talking on their phone all the time and started actually using the 5000 or 6000 rollover minutes they've collected.
It's dishonest, sure, but that's the way that these data networks are marketed almost everywhere, outside of a few places where data is billed on actual quantity used. I'm not saying AT&T
shouldn't have a network that can actually handle every smartphone user using at least 2GB of data per month. I'm just saying they
don't have one, and so, it's not a "free" proposition to them, that they can provide the tethering for free and it would not have a cost impact to them.
EDIT: If you really wanted to engage in civil disobedience around this, you could start a campaign of trying to get every smartphone user to use all the minutes / GB of data they pay for. If you could actually do it, you'd create a mess.