Does it matter whether the limit is physical or practical? On average, people use much less data when it's confined to a single phone than when they can share it with laptops.
Here's a different analogy that might work better: I can shop at Costco as much as I want, and I can even buy things for other people, but I cannot literally give my Costco card to someone else and let them shop at the store on their own.
Not when they are also charging you by the bit transferred.
People also deal with various data caps when tethering, they just now also deal with an extra bill for a service provided by their phone not the phone company.
Here's a different analogy that might work better: I can shop at Costco as much as I want, and I can even buy things for other people, but I cannot literally give my Costco card to someone else and let them shop at the store on their own.