Your right, it IS a service they offer. However charging me an additional fee to use my UNLIMITED DATA on a tethered device is completed horse crap. And honestly? I would encourage people to "steal this service." Maybe if enough people jump on the band wagon they will finally realize what total nonsense it is and start offering it for free instead of trying to get our money at every turn. Cell phone plans are already outrageously over priced... they don't need to screw us every chance they get.
I don't know about everyone else, but typically I liked to get kissed before I get effed.