I've never really understood this. If I'm paying for unlimited data, why does it matter how I choose to distribute it? What if i used the same amount of data on my phone as when I tether my iPad? It's flawed, greedy logic on their part. I know it's in the contract not to use it blah blah, but that doesn't mean it makes sense.
First, I would say that it should be one hundred percent illegal to sell an "unlimited data" plan that is not unlimited - it is fine with me to say have three data plans "low usage", "medium usage" and "high usage" at different cost and then specify somewhere _exactly_ what the user gets for each plan.
Second, it should be illegal to demand excessive payments for exceeding the limits of a data plan. I would find it acceptable to offer that data access is cut off at the limit, slowed down significantly when exceeding the limit, automatically update to the next higher plan or something similar, but a situation where a user can use twice the limit and pays ten times the price is unacceptable.
But now to your question: A company could offer, just as an example "up to four GB per month as fast as we can supply it, and then unlimited data at recduced speed of 100 kbit per second up to 50 MB per day". Many customers will take this offer. On average they will use a lot less than four GB per month. Because of that, the company can offer this deal at a price that is below the cost of actually providing 4 GB of data per month! If the average use is 1.3 GB per month, then the company can charge every user for the actual cost of 1.5 GB per month and make a good profit. If the company charged each of the users for the actual cost of 4 GB then they would make tons of money except that customers wouldn't buy it because it is too expensive.
The important point is that the cost to the end user can be lowered because the average use is much less than 4 GB. Now assume that people who don't tether use 0.9 GB on average and people who tether use 2.7 GB on average. And half the customers tether, so the average use is 1.8 GB. If the company offers one contract only (tethering allowed) they have to charge everyone for the 1.8 GB average use. If the company offers two different contracts (tethering allowed or not allowed, everything else is the same. Obviously the contract that doesn't allow tethering can be sold a lot cheaper than the one that allows tethering, because the average cost to the data provider for these contracts is lower.
Now you could say that maybe every user should be charged _exactly_ for the amount they use, which would make perfectly sense. Except that measuring the exact usage also costs money. Every time a little bit of data is moved to your phone the exact amount would have to be recorded somewhere. So the cost would now be fair, but a lot higher. The system as it is (different prices for tethered and untethered) is relatively cheap and reasonably, but not completely fair.
So you think you shouldn't pay more for tethering. Well, maybe a competitor offers _only_ a contract where tethering is always allowed, which is precisely what you want. That competitor would have to base the price on the cost for the average data use of _all_ its customers. Its plan would be in cost between the untethered and the tethered price of the first company. People using tethering would like the deal, people not using tethering would hate it. So this second company would initially get more customers who tether (like you). That means their average data use per user is higher, so the company needs to charge more, until only people who tether use it.
I've written to my members of Congress to get the FCC to step in and forbid extra charges for tethering. If I pay for 2 GB of data, it's irrelevant HOW I consume it. I've paid for it and it's mine to use as I see fit! The idea of charging ANY fee based on my method of connecting is asinine! If I go over my data limit, either with the phone OR notebook, I get charged for it. AT&T could actually MAKE money, legitimately, from excess data usage.
Ask them to change asinine way how car insurance works as well. I shouldn't pay for the risk that my car an my driving ability and style represents, but only for the damage that I actually cause. Wait a second... We don't need any car insurance at all, because that is exactly what you pay without insurance.
I don't think they know crap.. They are basing it on usage, and if you admit to them on the phone after your hate mail message. PLAY STUPID!!! Ask them when the last time you "tethered".... And to what device. What Mac address? If they can't answer.. Ask for someone who can..
Here is dangerous ground: AT&T can identify you (roughly, may not be hundred percent correct, and without having legal evidence of tethering and send you a message). You then either change your contract and pay for tethering, or you stop tethering and don't pay for it, or you can continue tethering and say "AT&T has no proof". In that third case, AT&T can and will collect and produce evidence, and they can then claim that you have been knowingly and even after their friendly reminder continued to breach the contract.
