All the buffets I know of advertise all you can EAT, and you don't need to sign anything (unless you pay by check or credit) because you agree to that contract once you pay and start eating. Stuffing food in your pockets is therefore a violation of that contract. Using T-Mobile's unlimited data on the other hand is NOT a violation of contract, because each party has a different idea of what is fair and reasonable which was not clearly established in the written contract. So your analogy doesn't hold.
You're also missing the actual customer base they're going after. It's people employing workarounds to get around tether caps. T-Mobile unlimited plans specifically have limited tethering, in various small denominations of gigabytes per month. IE: Unlimited phone usage, limited tethering usage of non-phone devices. So in this case, it is pretty well established in the paperwork you signed. For example, I get unlimited data and 5gb tethering on my "contract".
Whether or not they can detect people bypassing tethering restrictions by using third party software is another story altogether. But I suspect there are ways.