@briankrebs Prior to AT&T joining the GSM world, there were few if any differentiation between native use and tethering. Usage limits were very low anyways.
AT&T's negotiations for exclusive iPhone sales resulted in Apple providing different APN settings for tethering vs native use. Other handset makers followed and other carriers adopted it and became de-facto standard. This way, it is easy to block tethering at the APN level, restrict its speed etc while no impact on native use by handset.