hors modem, you are right. So France is different. I'm 100% sure that has never existed in Finland (well, the most common packages are unlimited anyway) and 90% sure it's not common in Germany either (they use throttling after you reach the limit)
So technically, how do they differentiate the traffic?
Most of the time, tethering is allowed in the base package. What happens afterwards depends. They can either cut you off or throttle. The only experience I have with this is Orange, and they throttle to the point where you basically have no connection (SSH is unusable).
Not sure though how they differentiate the traffic. There are "access points" set up which I presume are used for either connection. In my case they are the same but I seem to remember on an older phone they used to be different. It was also a different provider. I'm also not sure how they would detect that they're changed.
So technically, how do they differentiate the traffic?