Getting anything for free inwhich that your are supposed to pay for constitutes stealing. Period.
I'm sure as soon as they find out that you have been tethering with a phone on your plan and do not have tethering included, they will make you pay for the plan by adding it to your bill. And I'm sure if you try to have it removed, they will terminate your contract, shut you off, charge you early termination fees and backcharge you for all the tethering you have done on your last bill, then spit on you.
Of coarse, this is all speculation as I haven't tethered my phone to anything for this to come about but then again, I don't steal. So for now, assume all that you stated is correct and don't do it because all it does is make "them" either charge more fees or consider actual data caps on 3G/4G on us that do things the "legit" way....
Peace