> what does it matter what device generated the bits?
Most of the packages they sell are based not on the cost of your specific consumption (which is way more complex than peak bandwidth or total transfer), but the cost of typical consumption given some restrictions (e.g. the aforementioned bandwidth or total volume cap).
If you bunch tethered and non-tethered traffic together, the average non-tethered user will have to pay more, and the average tethered user will have to pay less, than with the two usages split (assuming the same total profit).
Thus, you would lose the "cheap" non-tethering customers to competitors who do differentiate, while the expensive tethering customers would come to you.
Or, if they limited bandwidth to the actual expected behavior, like the previous comment suggested, everyone would get the service they pay for regardless of device. The only ones who don't want that are the providers who want plans to be as confusing as possible.
Most of the packages they sell are based not on the cost of your specific consumption (which is way more complex than peak bandwidth or total transfer), but the cost of typical consumption given some restrictions (e.g. the aforementioned bandwidth or total volume cap).
If you bunch tethered and non-tethered traffic together, the average non-tethered user will have to pay more, and the average tethered user will have to pay less, than with the two usages split (assuming the same total profit).
Thus, you would lose the "cheap" non-tethering customers to competitors who do differentiate, while the expensive tethering customers would come to you.