umm... i disagree quite a bit with it actually, and others do as well.
It's been discussed to death, but just in case you're wondering, the argument FOR free tethering is: It's MY unlimited data, unlimited means UNLIMITED, I should be able to use it how I want.
The argument AGAINST free tethering (that I subscribe to) is: It's only unlimited because they're assuming you're using your phone, which at most will use a relatively small amount of data. If they thought you'd be using other devices which use much more data-- and take up more, actual bandwidth, which is a for realsies thing that costs them money-- they wouldn't have "unlimited" plans, at least not at the same price.
It's like tipping at a restaurant. The prices are lower because they expect you to tip the waitress. If they didn't expect you to tip, the cost of hiring servers would be reflected in the price of the food.
But.... we've discussed it to death, you decide what you want. No one's changing anyone's mind.