Here is an analogy for you.....
You have cable internet, your neighbor does not. So you run a splitter to his house so he can steal cable for free. How is tethering any different than that? Yes you pay for the cable but you don't pay to supply every Tom, Rich or Harry.
sent from the great depths of my phones internet (thanks Al Gore)
How is he stealing if you are giving something you yourself a paying for .
I have a well and give my neighbor water from that well it is mien to do so .
Now if I have city water and share the water and accept payment , and key word would be "accept payment" then it would be illegal .
If you tether and using it exclusively for your own purpose and not selling the data then you are not breaking any laws .
Maybe greedy corporate laws of greed , your getting a service and we are not exploiting you for it .