Incorrect, you would have to be getting free data for it to be stealing. As in not paying for it. You are paying for it, however it is simply breaking your promise to not do with it what you said. Breaking an agreement is not stealing.
This is like your cable company putting in it's ToS : "Customer may not watch our cable TV service on any TV bigger than 32 inches without paying an additional 20 dollars". If you went ahead and watched cable TV on a 42" without paying, what are you stealing from the cable provider? The answer is nothing, you're just breaking the agreement.
Tethering for free is the same thing. You are paying for service, said service includes unlimited data, or 2gb, what have you. You agree to not use that data on anything other than your phone. You take nothing from the provider if you do, you just break your word.
This should be crystal clear to all except those who can't be reasoned with.
Tethering is not a service of your provider, it is a function of the device.