How Long Before Phone Carriers Stop Charging Fees for Additional Devices?

It's $10 for me to add a device to my shared data plan, but it cost zero $ to use a personal hotspot. Why do they charge a fee? It seems silly to me. It reminds of of when wireless routers were first being introduced and cable companies wanted to charge its customers additional fees if you had more than 1 computer connected.

I paid for a certain amount of data...why should I pay more if I want to use multiple devices to access it? It cost the phone company nothing in overhead for the additional devices.