No it is a good analogy. Your analogy is a bad one.
Unlimited data is like an all you can eat buffet. That does not mean that you can share your food with someone else. It is all YOU can eat. You being the phone. Your computer is someone else and that no place would let you do.
Tethering your phone to your computer has increased data usage and as such throws off the models they use for how the network is supported. If you pay for tethering that is factored in differently and accounted for and the model is adjusted as such. Other wise the model is screwed up because some people are tethering but not paying for it so it hurts everyone but the ones tethering and getting a free ride.
The massive data hogs are a problem for the network as they are outliers in the the system and as such really throw off the statistical models.
