The solution to this is to give free tethering to everyone, but get rid of any unlimited plans, and continue to tier based on GB. That way there's no problem as to who used what for what. You pay $25, you get 2GB, more GB are $10/GB. That and they should offer the customer the option to throttle instead of overage charges. That would be unlimited, just after 2GB you only get maybe 200kbps.
You are still missing the point. If you throttle you are limiting.
If you limit some one to a speed that equates to a download of 1MB per second. During the period of the next hour you have limited their download to 3.6GB.
The solution to this is to not use the term unlimited.