Hence they word 'analogy'. duh.
They are however both products that you may buy and consume.
Perhaps another analogy you would understand better. If your landline ISP enforces a cap on your usage, would you think it reasonable for them to charge you extra for additionally using your iPhone on your home network and not just your Mac?
The best reasoning for accepting the extra tethering fee on a capped plan is that if you tether, you are more likely to use the data that you paid for. If you don't tether you likely will pay for data you are not using. So the argument is that if you are more likely to actually use what you have paid for then you should pay an extra fee. In the analogy above, you having your family's iPhones and iPads and Mac on your home network makes it more likely that you will use more of the data that you have paid for than if you just had your own Mac on the network. If you don't exceed your cap, would you accept your ISP saying they are going to start charging you almost twice your monthly fee?
"My 8th grade math teacher once said: "You can't help it if you're dumb, you are born that way. But stupid is self inflicted."" -Hiro.