Cellular and mobile broadband providers should not be able to get away with this. If you advertise a service for "Unlimited" data, that therefore means that they will get as much data as they want, with the best data rate possible.
This also means that you will never cap usage daily, weekly, monthly, yearly or for the life of their service.
If I have an unlimited plan, a truly unlimited plan, not "unlimited... up to 5Gb a month" (that's limited), then I should be able to get as much data as I want. If I tether (Wi-Fi, Bluetooth, USB, or any other data transfer protocol) my cellphone to my computer (or any other internet-connected device) you cannot charge me another plan OR data usage, if I am already paying for "unlimited" data service. Ultimately if I were to pay for a "tethered device" plan, or you charge subsequent fees for using a device connected to my cellular modem, you are charging me TWICE for the same service. Electric utilities do not charge differently whether the electricity is going to a Plasma TV, heater, or a fridge, they provide a service for a certain rate, and you are no exception.