Paying for tethering is the digital equivalent of Shell or BP saying, "unlimited diesel for your compact car for $X/month. Add your 4mpg Hummer for only $Y/month additional!"
that's a very bad analogy. Apps waste way more bandwidth. It's better if you turn it around: "unlimited gas for you 4mpg hummer. But we will nickel and dime you if you try to ever fill up the 1L reserve tank on your electric scooter"
the tethered devices use the same "gas tank" and would have to be directly connected at all times for that to work.
so maybe
"you get unlimited diesel for this car, but you'll have to pay extra if you want to charge your phone or laptop from the electricity generated from our diesel"
