Really?, Which country are you talking about? Here is Australia, I can tether with Aldi Mobile, a reseller of the Telstra mobile network. Works fine in Android and iPhone.

The US.

Tethered traffic is usually routed differently. Almost all cellular data traffic is CGNATed or proxied, and different priority is assigned to different types of traffic.

For 95% of users, there’s no problem at all. When people push the limits, they find themselves in a pickle. I’m familiar with an organization with >50k devices across 4 major carriers 4-5 years ago. They probably had <500 people who required some sort of exception, ranging from a different plan to a more appropriate device and plan, to someone doing something crazy. (One guy was running a small field office off of a Samsung tablet)