I agree with you that it's a bad analogy. But to be fair, all-you-can-eat buffets are not really all-you-can-eat, and will kick you out if you try to consume more than what they calculated a regular consumer does. All service providers do this when they falsely advertise "unlimited" anything.

Where ISPs cross the line is by trying to also enforce _how_ I can consume the data I'm paying for. Having plans that restrict tethering is consumer-hostile, plain and simple. Whether I'm tethering or not has no relation to how much data I consume. They can continue to restrict bandwidth and data limits if I go overboard, but I'll be damned if I allow them to tell me how I can use it.

So if we're going with the buffet analogy, then it's like them saying I can only use a fork to eat, as someone mentioned above.