Carriers in the US configure phones so that the tethering APN is always used for tethering specifically over all interfaces, the traffic is also tagged with a different TTL. They also do some malarkey with deep packet inspection (in case you figure out how to modify the APN) to identify if a computer is using the connection (like the initial phone home Windows and Mac both do to determine the type/quality of Internet connection they are connected to.)

All of this one can work around, but it becomes an annoying game of cat and also cat, and then if the carrier “decides” you violated the “spirit” of their TOS, they’ll cancel your plan and take your number away.