This is maddening to hear. The cellular companies need to figure out what product they are gonna sell. If they are selling an internet connection - what do they care if there is 1 or 100 devices on the other end? They limit the connection, so you cannot consume “more” internet than what you paid for; and they also oversubscribe their network - so, you typically can expect much “less” internet than what you paid for.
I am surprised people don’t set up their network to 100% utilize the links they pay for. i.e. Have a VM doing a speedtest on a loop and that data has the lowest priority (speedtest gets slowed down when congestion occurs). If you pay for a 10Mbps connection - you should be able to keep that thing downloading at 10Mbps all day/night/week/month if you wanted. That is what they sold you.
The part that bugs me the most about data caps is that there is nothing the consumer can do to limit the amount of data that gets returned from a web request. If google decides to put a 2GB image on their homepage - what is the consumer supposed to do about it?
Charging more for tether devices makes about as much sense as the water company charging you different rates if you have a shower vs a bathtub.