It's also because U.S. carriers don't like people hooking up arbitrary devices that can run arbitrary software to their network. In the civilized world, you have a device that talks GSM/LTE, you're golden as long as you don't violate any transmission laws. But in the USA carriers are still doing device allowlisting because I guess they want to bin QoS and don't want pro-grade traffic going over consumer accounts, nor the added expense of support for consumer accounts with exotic hardware that "might" break the network.