The issue is the lack of cheap phone plans that allow tethering. And even the more expensive ones that do often have low data caps.
On the "how they detect it", unless you're using a VPN they can look at the traffic and easy distinguish "computer traffic" from regular "phone traffic".
On the mobile hotspot settings page it will just show a message with a link to the carriers website instead of the toggle to enable it, I find it really annoying since as you said I don't understand how there would be a difference in what the usage would look like to the carrier and it feels like Apple is imposing a restriction just for the benefit of the carrier that makes the phone less usable.