I used to do consulting for an international telecom....
One telco customer did not have tethering limitations in their customer contract agreements, so this one end users customer bought a number of devices, and then would resell internet service in local rural areas using his devices as backhaul. This one customer and his dozens of "ISP" devices were doing gigabytes and gigabytes of data per month, using approximately 80% of this whole carrier's data capacity.
One telco customer did not have tethering limitations in their customer contract agreements, so this one end users customer bought a number of devices, and then would resell internet service in local rural areas using his devices as backhaul. This one customer and his dozens of "ISP" devices were doing gigabytes and gigabytes of data per month, using approximately 80% of this whole carrier's data capacity.