It's simple you enter a contract not to tether, if you do you break the contract terms. Up to you if you do so, but then also up to the carrier to charge you extra or terminate the contract.
You should be able to tether to another device without paying extra. If I have unlimited data on my phone and I want to use my iPad I should be able to do so. Tethering several devices or a desktop/laptop would be wrong. But to tether to another iOS device shouldn't be a big deal. I wouldn't even mind if they locked the phone while in tether mode to ensure I couldn't use more than one device at a time.