I think it's unethical for carriers to even have a tethering plan.
It's my data that I paid for and it's my phone's function that I bought, what difference does it make to the carrier?
I'll explain it to you.
The carrier offers a deal. The carrier estimates what the average cost of that deal will be, and sets the price accordingly. Usually, the carrier offers a choice of different deals, and they will have different costs and therefore different prices. Each deal will attract some set of customers.
Now a carrier might offer two deals: Unlimited without tethering, and unlimited with tethering. They cost different amounts of money. Why? Because people on a "without tethering" plan use less data on average than the people on a "with tethering" plan. Tthe cost to the carrier depends on the _average_ use of all users on that plan, and so does the price that is set. People on the "with tethering" plan will use more data on average so the cost is higher.
If you use the "without tethering" plan and tether, then you are likely to use more bandwidth than the average. Therefore, you increase the cost for the carrier. Therefore, the price goes up. So you are punishing all the people who don't tether, because they are going to pay for the additional data that you will use by tethering.