Its not a case of how you use your data, its a case of youre not supposed to use your phone as a modem unless you have a plan that allows you to do so. Fine print that you agree to says something along the lines of "I will adhere to the terms of my plan and yada yada." In which case a $30 unlimited data plan is not a tethering plan, but the $60 unlimited data plus tethering plan does include tethering.
now believe me, ive done it and still do occasionally bur I understand why they do it.
I understand they have every legal right to do that, I don't contest that at all, its the fact that I'm buying something (data) and now I have to buy permission to use the data how I please, same up/down rates, same coverage, just a permission, and that's bull s*** if you ask me,
Apply this to any other product, I buy a car, its ultimately mine to do with whatever I please, then the dealership calls you and says "you can't drive on the southside unless you pay another 10k"