For two people to work, watch movies, listen to music, podcasts for a month?
It's stupid because I'm paying for the data, why should a carrier get a say in saying it has to be used from my iPhone and not my Mac? Totally asinine, on my UK contract that costs a fraction of a T-Mobile plan it's *actually* unlimited because we have consumer protections over there and I can tether all day without restrictions.
Think of it as a company giving you unlimited fuel for your car for a fixed monthly fee.
So you start siphoning fuel out of your car to run a generator 24/7 at home to power your house.
The company entered in the agreement with you with the understanding that they'd be paying for fuel used by the car. Some people use more than they're paying for, some people use less. But overall it works out on average to make the company money.
Running the generator will use a lot more fuel and suddenly you're losing the company money. If everyone does it...
It's the same idea with tethering. A Mac tethered to a phone uses a lot more bandwidth than the phone typically would on its own. Multiply that if you have your whole household tethering. It's not sustainable. If they couldn't differentiate, they wouldn't be able to offer the unlimited plan.
Believe me, I used to think your way too. There's a lot more nuance to this stuff though.